Liothyronine sodium

What it is

Liothyronine sodium is a synthetic copy of triiodothyronine, the active thyroid hormone, supplied as tablets dosed in micrograms. It is almost completely absorbed, about 95 percent within four hours, and has a biological half-life of about two and a half days, much shorter than levothyroxine. In the body most T3 is normally made from T4 by enzymes in peripheral tissues rather than taken directly.

What the research says

T3 is the form of thyroid hormone that actually acts on cells, so giving it works biochemically: trials consistently show higher blood T3 and lower T4. Whether that makes people feel better than levothyroxine alone is the contested part. Meta-analyses of randomised trials have repeatedly found no difference in quality of life, mood, fatigue, weight or cholesterol, at low to moderate certainty, while more patients say they prefer the combination. Safety data are reassuring: a 2025 meta-analysis of cohort studies covering 630,254 people found no increase in atrial fibrillation, heart failure or stroke. The boxed warning is about misuse for weight loss, not about replacement.

Evidence grade: Disputed.

How it works

Drug class: Thyroid hormone (synthetic L-triiodothyronine, T3)

T3 enters the cell nucleus, binds thyroid hormone receptors sitting on DNA, and switches on genes - that is how thyroid hormone sets the pace of metabolism in almost every tissue. Nearly all of thyroid hormone’s actual effect is done by T3; about 80 percent of the T3 in your blood is normally made by stripping an iodine atom off T4 out in the tissues. Taking T3 directly skips that conversion step, which is why it acts within hours. (Source 1)

Boxed warning

WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS

(Source 2)

What it is used for

• Thyroid hormone replacement for hypothyroidism is not in doubt, but liothyronine as an addition to or substitute for levothyroxine has not outperformed levothyroxine alone in pooled randomised trials on symptoms, quality of life or fatigue. Evidence: established. (Source 3)
• The label approves this use and says the dose should target a TSH range. We found no outcome trial of liothyronine specifically for TSH suppression in thyroid cancer in our searches. Evidence: unknown. (Source 3)
• The label describes the test: 75 to 100 micrograms daily for 7 days with radioiodine uptake measured before and after, where 50 percent or greater suppression indicates a normal thyroid-pituitary axis. We found no modern validation study of this test. Evidence: unknown. (Source 4)
• A 1996 meta-analysis of eight studies found patients twice as likely to respond, but among the four randomised double-blind studies the effect was not significant. A 2026 meta-analysis with bias-adjustment found T3 augmentation gave no significant advantage over control across six trials. Evidence: disputed. (Source 5)
• This is the subject of the boxed warning. In people with normal thyroid function, replacement-range doses do not cause weight loss, and larger doses can be seriously or fatally toxic, particularly combined with stimulant appetite suppressants. Evidence: not-supported. (Source 2)

Interactions

• Bile acid sequestrants (cholestyramine, colesevelam, colestipol) and ion-exchange resins (sevelamer, Kayexalate) (label): These bind thyroid hormone in the gut and delay or prevent its absorption, which can leave someone under-treated. The label asks for liothyronine at least 4 hours earlier, or TSH monitoring. (Source 6)
• Calcium, iron and soy supplements (label): A documented gap rather than a documented interaction. The only absorption-reducing agents the liothyronine label lists are bile acid sequestrants and ion-exchange resins; calcium, iron and soy do not appear in it, and we found no liothyronine-specific absorption study in our searches. The calcium, iron and soy evidence familiar from thyroid care is for levothyroxine, a different molecule with different protein binding. (Source 6)
• Biotin supplements (case reports): Biotin does not change what liothyronine does, but it corrupts the blood tests used to adjust the dose. Because biotin is built into many immunoassays, supplement doses can make thyroid results read falsely high or falsely low, and a published case was misdiagnosed as Graves’ disease. (Source 7)
• Warfarin and other oral anticoagulants (label): Liothyronine increases the effect of oral anticoagulants, so correcting an underactive thyroid or raising the liothyronine dose can push clotting tests too far and may require the anticoagulant dose to come down. (Source 8)
• Sympathomimetics, including stimulant appetite suppressants (label): This is the combination the boxed warning singles out. Each can amplify the other, and thyroid hormone plus a sympathomimetic raises the risk of inadequate blood supply to the heart in people with coronary artery disease. (Source 8)
• Tricyclic and tetracyclic antidepressants (label): Each can increase both the wanted and the toxic effects of the other, including heart rhythm disturbance and central nervous system over-stimulation. Liothyronine may also bring a tricyclic’s effect on faster - which is the basis of the off-label augmentation use. (Source 8)
• Digoxin and other digitalis glycosides (label): Liothyronine may reduce the effect of digitalis drugs, and digitalis blood levels can fall as an underactive thyroid is corrected. (Source 8)
• Ketamine (label): Taken together they may cause marked high blood pressure and a fast heart rate. (Source 8)

Stopping it

• There is no tapering schedule in the label, and we found no deprescribing or withdrawal trial for liothyronine. What the label does describe is the switch between preparations: because liothyronine acts within hours while the previous drug lingers for weeks, levothyroxine is stopped and liothyronine started low and increased gradually. (Source 4)
• Liothyronine clears faster than levothyroxine, so blood levels fall sooner after a missed or stopped dose. The label puts its biological half-life at about two and a half days, against the longer half-life it ascribes to levothyroxine. (Source 1)
• Stopping it is not dangerous in the way stopping some drugs is: the 2025 safety review found reported adverse events clustered in unregulated use and compounding errors rather than in regulated therapy. The consequence of stopping is the return of hypothyroidism, not a withdrawal syndrome. (Source 9)

What goes wrong

The boxed warning on every liothyronine label we checked is that it must not be used for obesity or weight loss. (Source 2)

• Official position, Certainty not rated.
• Size: not stated.
• Who: euthyroid people and people given thyroid hormone for weight reduction.
• How long: not stated.
• Result: in euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction; larger doses may produce serious or life-threatening toxicity, particularly with sympathomimetic amines used as appetite suppressants.
• Funding: not applicable (FDA-approved labelling)

Thyroid hormones, including CYTOMEL, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. • In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction.

The label’s adverse reactions are the symptoms of too much thyroid hormone, and include reduced bone mineral density and cardiac events. (Source 10)

• Official position, Certainty not rated.
• Size: not stated.
• Who: patients treated with liothyronine.
• How long: not stated; the label attributes these primarily to therapeutic overdosage.
• Result: no rates given. Cardiovascular: palpitations, tachycardia, arrhythmias, raised pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest. Endocrine: decreased bone mineral density. Also weight loss, heat intolerance, tremor, insomnia, hair loss, menstrual irregularity and impaired fertility.
• Funding: not applicable (FDA-approved labelling)

Adverse reactions associated with CYTOMEL therapy are primarily those of hyperthyroidism due to therapeutic overdosage [see Warnings and Precautions (5.4) and Overdosage (10)]. They include the following: General: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating Central nervous system: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia Musculoskeletal: tremors, muscle weakness and cramps Cardiovascular: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest Respiratory: dyspnea Gastrointestinal: diarrhea, vomiting, abdominal cramps, elevations in liver function tests Dermatologic: hair loss, flushing Endocrine: decreased bone mineral density Reproductive: menstrual irregularities, impaired fertility

A case report combined with a systematic review of the literature found that biotin supplements can make thyroid blood tests read wrongly, which has led to a false diagnosis of overactive thyroid. (Source 7)

• Systematic review, Low certainty.
• Size: one reported patient plus the published literature identified by the authors’ systematic review.
• Who: a patient taking biotin with markedly abnormal thyroid function tests.
• How long: tests normalised after biotin was stopped, faster than the half-life of T4 allows.
• Result: no rate given. Interference can be falsely high or falsely low depending on assay format, and affects analytes beyond thyroid tests.
• Funding: not stated.

Limit of this finding: The general statements here - that interference can be falsely high or falsely low and reaches beyond thyroid tests - rest on the authors’ review of published reports, not on their single patient.

Biotin ingested in moderate to high doses can cause immunoassay interference. Depending on the assay format, biotin interference can result in either falsely high or low values. Interference is not limited to thyroid tests and has the potential to affect a wide range of analytes.

What the evidence supports

One double-blind randomised crossover trial that held TSH constant between arms did find an advantage for combination therapy, which is the main dissenting trial. (Source 11)

• Randomized trial, Low certainty.
• Size: 59 patients, 55 women, median age 46.
• Who: patients with hypothyroidism on stable levothyroxine with TSH 0.1 to 5.0 mU/l for six months.
• How long: 12 weeks per arm with crossover, 24 weeks total.
• Result: significant differences favouring combination in 7 of 11 quality-of-life and depression scores; 49% preferred the combination and 15% monotherapy (P=0.002); TSH unchanged between groups. 50 micrograms of the usual T4 dose was replaced by either 20 micrograms T3 or 50 micrograms T4.
• Funding: not stated.

Limit of this finding: The preference figures leave a gap: 49% preferred the combination and 15% preferred monotherapy, which means about a third of patients expressed no preference. The quoted sentence does not say so, so do not read it as a straight split of everyone in the trial.

When comparing scores of QOL and depression on T(4) monotherapy versus T(4)/T(3) combination therapy, significant differences were seen in 7 out of 11 scores, indicating a positive effect related to the combination therapy. Forty-nine percent preferred the combination and 15% monotherapy (P=0.002). Serum TSH remained unaltered between the groups as intended.

A 2025 multisource review of safety found no increase in atrial fibrillation, heart failure or stroke on liothyronine, and lower mortality in observational cohort data only. (Source 9)

• Systematic review, Low certainty.
• Size: 52 studies (21 RCTs, 4 cohort studies, 27 case reports); RCT meta-analysis n = 2,128; cohort meta-analysis n = 630,254.
• Who: liothyronine users compared with levothyroxine-only users.
• How long: varies; not stated in the abstract.
• Result: adverse events combination vs monotherapy RR 1.22 (95% CI 0.66-2.25); atrial fibrillation RR 1.10 (0.74-1.63); heart failure RR 1.54 (0.95-2.47); stroke RR 0.86 (0.11-6.75); mortality RR 0.70 (0.62-0.78). Reported adverse events were confined to unregulated use or compounding errors. The mortality finding comes from observational data, so it cannot be read as liothyronine preventing death.
• Funding: not stated.

Limit of this finding: The mortality figure, RR 0.70 (0.62 to 0.78), comes from observational cohorts, not from randomised trials. People prescribed liothyronine differ from those who are not - in age, in why they were prescribed it, in how closely they are followed - so this cannot be read as liothyronine preventing deaths. The rhythm, heart-failure and stroke results are reassuring but their confidence intervals are wide.

LT3-related AEs were only reported with unregulated LT3 use or pharmacy compounding errors. LT3 and LT4 showed similar adverse severity profiles in the Yellow Card scheme. Disproportionality analysis in the FAERS database showed no increased LT3 safety signals. A meta-analysis of RCTs (n = 2128) showed a similar AEs risk for combination vs monotherapy [relative risk (RR) 1.22, 95% confidence interval (CI) 0.66-2.25]. A cohort study meta-analysis (LT3 vs LT4-only users, n = 630 254) showed no increased risk of atrial fibrillation (RR 1.10, 95% CI 0.74-1.63), heart failure (RR 1.54, 95% CI 0.95-2.47), or strokes (RR 0.86, 95% CI 0.11-6.75), but reduced mortality risk was observed for LT3 (RR 0.70, 95% CI 0.62-0.78).

What the evidence does not support

Pooled randomised trials found no symptom or quality-of-life advantage for T4 plus T3 over T4 alone. (Source 12)

• Meta-analysis, Moderate certainty.
• Size: 11 randomised trials, 1,216 patients.
• Who: adults with clinical hypothyroidism.
• How long: varies by trial; not stated in the abstract.
• Result: bodily pain SMD 0.00 (95% CI -0.34, 0.35); depression SMD 0.07 (-0.20, 0.34); anxiety SMD 0.00 (-0.12, 0.11); fatigue SMD -0.12 (-0.33, 0.09); quality of life SMD 0.03 (-0.09, 0.15); no difference in body weight, total cholesterol, triglycerides, LDL or HDL; adverse events did not differ.
• Funding: not stated.

We included 11 studies, in which 1216 patients were randomized. No difference was found in the effectiveness of combination vs. monotherapy in any of the following symptoms: bodily pain [standardized mean difference (SMD) 0.00, 95% confidence interval (CI) -0.34, 0.35], depression (SMD 0.07, 95% CI -0.20, 0.34), anxiety (SMD 0.00, 95% CI -0.12, 0.11), fatigue (SMD -0.12, 95% CI -0.33, 0.09), quality of life (SMD 0.03, 95% CI -0.09, 0.15), body weight, total serum cholesterol, triglyceride levels, low-density lipoprotein, and high-density lipoprotein. Adverse events did not differ between regimens.

A later systematic review reached the same conclusion at low-to-moderate certainty, while finding that more patients prefer the combination. (Source 13)

• Systematic review, Moderate certainty.
• Size: 18 studies from 1,398 references screened.
• Who: adult patients with hypothyroidism.
• How long: not stated in the abstract.
• Result: no difference in clinical status, quality of life, psychological distress, depressive symptoms or fatigue; patient preference 43% combined vs 23% monotherapy vs 30% no preference; adverse events similar but assessed narratively only.
• Funding: not stated.

Results supported by evidence at low-to-moderate certainty evidence did not display a difference in treatment effect between therapies on clinical status, quality of life, psychological distress, depressive symptoms, and fatigue; all measured with standardized questionnaires. Furthermore, meta-analysis of patient preferences revealed higher proportions of choice for combined therapy (43%) when compared with monotherapy (23%) or having no preference (30%). When evaluating treatment adverse events or adverse reactions, similar proportions were observed between treatment groups; meta-analysis was not possible.

A 2026 meta-analysis with publication-bias adjustment found T3 augmentation gave no significant response advantage in major depression. (Source 14)

• Meta-analysis, Moderate certainty.
• Size: 56 RCTs, 13,616 participants in total; 6 trials for the T3 comparison.
• Who: adults with major depressive disorder responding inadequately to antidepressant treatment.
• How long: not stated in the abstract.
• Result: T3 augmentation response OR 1.19 (95% CI 0.53-2.70) across 6 trials, not significant; by contrast second-generation antipsychotics OR 1.53 (1.42-1.65)
• Funding: not stated.

Additionally, T3 augmentation was not associated with significantly higher response than controls (k = 6; 1.19; 0.53-2.70). Among individual SGAs, only aripiprazole, quetiapine, brexpiprazole, and cariprazine demonstrated TSA-supported efficacy, whereas evidence for other SGAs was limited or inconclusive.

Where the evidence is mixed

A 2024 meta-analysis found the expected blood-test changes from adding T3 but no difference in heart rate, lipids or depression scores. (Source 15)

• Meta-analysis, Low certainty.
• Size: 16 studies from 6,394 records; 15 on combined therapy, 2 on desiccated thyroid.
• Who: adults with hypothyroidism.
• How long: not stated in the abstract.
• Result: free T4 MD -0.34 (95% CI -0.47, -0.20); total T4 MD -2.20 (-3.03, -1.37); total T3 MD 29.82 (22.40, 37.25); GHQ-28 MD -2.89 (-3.16, -2.63); no significant difference in heart rate, SHBG, TSH, lipid profile, TSQ-36 or BDI; moderate to high heterogeneity.
• Funding: not stated.

Limit of this finding: Two things in this abstract do not hold together. It says 16 studies qualified and then describes 2 studies of desiccated thyroid plus 15 of combination therapy, which is 17. And the mean differences for Free T4 (-0.34) and Total T4 (-2.20) are printed with no units, so they cannot be interpreted on their own; that omission is the paper’s, not a lost unit on our side.

In this meta-analysis, combination therapy with T4 + T3 revealed significantly lower Free T4 levels (mean difference (MD): -0.34; 95% CI: -0.47, -0.20), Total T4 levels (mean difference: -2.20; 95% CI: -3.03, -1.37), and GHQ-28 scores (MD: -2.89; 95% CI: -3.16, -2.63), compared to T4 monotherapy. Total T3 levels were significantly higher in combined therapy (MD: 29.82; 95% CI: 22.40, 37.25). The analyses demonstrated moderate to high heterogeneity. There was no significant difference in Heart Rate, SHBG, TSH, Lipid profile, TSQ-36, and BDI Score.

A 17-year population study detected no excess cardiovascular disease, atrial fibrillation or fractures on liothyronine, but more than double the rate of new antipsychotic prescriptions. (Source 16)

• Cohort study, Low certainty.
• Size: 400 patients who had ever used liothyronine compared with 33,955 who had only used l-thyroxine.
• Who: everyone prescribed thyroid hormone replacement in Tayside, Scotland (population about 400,000)
• How long: 1997 to 2014.
• Result: cardiovascular disease HR 1.04 (95% CI 0.70-1.54); atrial fibrillation HR 0.91 (0.47-1.75); fractures HR 0.79 (0.49-1.27); new antipsychotic prescriptions HR 2.26 (1.64-3.11, P < 0.0001), proportional to number of liothyronine prescriptions; non-significant trend towards more breast cancer and antidepressant use; median TSH 1.07 on liothyronine vs 2.08 on levothyroxine alone.
• Funding: not stated; observational, so prescribing by indication is a strong alternative explanation for the antipsychotic signal.

Limit of this finding: Only 400 people in this study had ever taken liothyronine, against 33,955 on levothyroxine alone, so the reassuring results are imprecise: an atrial fibrillation hazard ratio of 0.91 with a range of 0.47 to 1.75 is compatible with a meaningful increase or decrease. Read these as "no increase was detected", not as "there is no increase". The one positive result, more new antipsychotic prescriptions, comes from prescribing records in an observational dataset, so the obvious competing explanation is why liothyronine was prescribed in the first place.

Compared to patients only taking l-thyroxine, those using liothyronine had no increased risk of cardiovascular disease [hazard ratio (HR) 1·04; 95% CI 0·70-1·54], atrial fibrillation (HR 0·91: 0·47-1·75), or fractures (HR 0·79: 0·49-1·27) after adjusting for age. There was no difference in the number of prescriptions for bisphosphonates or statins. There was an increased risk of new prescriptions for antipsychotic medication (HR 2·26: 1·64-3·11 P < 0·0001) which was proportional to the number of liothyronine prescriptions.

The older meta-analysis that made T3 augmentation popular lost its significance once only the randomised double-blind studies were counted. (Source 5)

• Meta-analysis, Low certainty.
• Size: 8 studies, 292 patients; 4 of them randomised and double-blind.
• Who: euthyroid, non-psychotic patients with depression refractory to tricyclic antidepressants.
• How long: not stated in the abstract.
• Result: all 8 studies: relative response 2.09 (95% CI 1.31-3.32, P = .002), absolute improvement in response rates 23.2% (4.5% to 41.9%, P = .02), standardized effect size 0.62 (P < .001). The 4 randomised double-blind studies: relative response 1.53 (0.70-3.35, P = .29). The authors describe study quality as uneven and results as statistically heterogeneous.
• Funding: not stated.

Aggregating eight studies with a total of 292 patients, patients treated with triiodothyronine augmentation were twice as likely to respond as controls (relative response, 2.09; 95% confidence interval [CI], 1.31 to 3.32; P = .002). This corresponded to a 23.2% absolute improvement in response rates (95% CI, 4.5% to 41.9%; P = .02). Improvements in depression scores were moderately large (standardized effect size, 0.62; P < .001). However, study quality was uneven, and results were statistically heterogeneous. Among the four randomized double-blind studies, pooled effects were not significant (relative response, 1.53; 95% CI, 0.70 to 3.35; P = .29), but one study with negative results accounted for most of the intertrial heterogeneity in results.

Where the research disagrees

Whether adding liothyronine to levothyroxine makes patients feel better

• Grozinsky-Glasberg and colleagues, meta-analysis of 11 randomised trials in 1,216 patients, meta-analysis of randomised controlled trials: No difference was found in the effectiveness of combination vs. monotherapy in any of the following symptoms: bodily pain [standardized mean difference (SMD) 0.00, 95% confidence interval (CI) -0.34, 0.35], depression (SMD 0.07, 95% CI -0.20, 0.34), anxiety (SMD 0.00, 95% CI -0.12, 0.11), fatigue (SMD -0.12, 95% CI -0.33, 0.09), quality of life (SMD 0.03, 95% CI -0.09, 0.15) (Source 12)
• Nygaard and colleagues, double-blind randomised crossover trial in 59 patients with TSH held constant, randomised double-blind crossover trial: significant differences were seen in 7 out of 11 scores, indicating a positive effect related to the combination therapy. Forty-nine percent preferred the combination and 15% monotherapy (P=0.002). (Source 11)
• Millan-Alanis and colleagues, systematic review of 18 studies, systematic review and meta-analysis with certainty rating: Furthermore, meta-analysis of patient preferences revealed higher proportions of choice for combined therapy (43%) when compared with monotherapy (23%) or having no preference (30%). (Source 13)

Whether T3 augmentation helps treatment-resistant depression

• Aronson and colleagues, 1996 meta-analysis of 8 studies, meta-analysis, mostly of non-randomised or unblinded trials: patients treated with triiodothyronine augmentation were twice as likely to respond as controls (relative response, 2.09; 95% confidence interval [CI], 1.31 to 3.32; P = .002). This corresponded to a 23.2% absolute improvement in response rates (95% CI, 4.5% to 41.9%; P = .02). (Source 5)
• Hsu and colleagues, 2026 meta-analysis with bias-adjustment analyses, systematic review and meta-analysis with trial sequential and publication-bias adjustment: T3 augmentation was not associated with significantly higher response than controls (k = 6; 1.19; 0.53-2.70). (Source 14)

Whether long-term liothyronine is cardiovascularly risky

• Bahl and colleagues, multisource systematic review including 630,254 people in cohort studies, systematic review and meta-analysis of RCTs, cohorts and pharmacovigilance databases: A cohort study meta-analysis (LT3 vs LT4-only users, n = 630 254) showed no increased risk of atrial fibrillation (RR 1.10, 95% CI 0.74-1.63), heart failure (RR 1.54, 95% CI 0.95-2.47), or strokes (RR 0.86, 95% CI 0.11-6.75) (Source 9)
• The CYTOMEL label, listing class adverse reactions, FDA-approved labelling, adverse reactions section: Cardiovascular: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest (Source 10)

How much

• Reference intake: Dosing is set by the prescriber. As a position, the US CYTOMEL label (SPL effective 2026-05-18) gives, under "Adults", a starting dose of 25 micrograms once daily for hypothyroidism, increased by 25 micrograms every one or two weeks if needed, with a usual maintenance dose of 25 to 75 micrograms once daily, and a 5 microgram start for elderly patients or those with heart disease. The paediatric regimen is separate and lower: 5 micrograms once daily to start, increasing by 5 micrograms every 3 to 4 days. Note the unit: these are micrograms, a thousandth of a milligram. (Source 17)
• Upper limit: The label does not state a maximum dose for hypothyroidism; it gives a usual maintenance range of 25 to 75 micrograms once daily and says TSH suppression in thyroid cancer may require higher doses. The highest fixed dose it specifies anywhere is the diagnostic thyroid suppression test, 75 to 100 micrograms daily for 7 days. (Source 4)
• Studied: A double-blind randomised crossover trial replaced 50 micrograms of the usual levothyroxine dose with either 20 micrograms of T3 or 50 micrograms of T4 for 12 weeks each. (Source 18)
• Studied: The pooled meta-analysis of 11 trials in 1,216 patients compared T4-T3 combination against T4 monotherapy; the abstract does not report a single pooled dose. (Source 12)

A common belief, and what the research shows

The belief: T3 is the active thyroid hormone, so switching to it or adding it should work better than levothyroxine for anyone still feeling unwell.

What the research shows: It is true that T3 does the work - the label says "The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues." But feeling better is a separate question, and the randomised evidence has not shown it. A meta-analysis of 11 trials in 1,216 patients found "No difference was found in the effectiveness of combination vs. monotherapy in any of the following symptoms: bodily pain [standardized mean difference (SMD) 0.00, 95% confidence interval (CI) -0.34, 0.35], depression (SMD 0.07, 95% CI -0.20, 0.34)". A later review at low-to-moderate certainty agreed, while noting that "meta-analysis of patient preferences revealed higher proportions of choice for combined therapy (43%) when compared with monotherapy (23%) or having no preference (30%)". The second big misconception is weight: the boxed warning states that "In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction."

Questions and answers

What is it?

Liothyronine sodium is a man-made copy of T3, the active thyroid hormone, taken as a tablet. Doses are in micrograms, not milligrams. Almost all of it is absorbed - about 95 percent within four hours - and it is cleared with a biological half-life of roughly two and a half days. (Source 1)

What does it do in the body?

T3 goes into the cell nucleus, binds receptors attached to DNA and switches genes on, which sets the metabolic rate of nearly every tissue. Nearly all of thyroid hormone’s effect is carried by T3, and about 80 percent of your T3 is normally made from T4 in the tissues rather than coming from the thyroid directly. (Source 1)

Is it good or bad for you?

Good as replacement when the thyroid cannot supply enough hormone; unproven as an upgrade on levothyroxine; dangerous as a weight-loss drug. Pooled trials found no symptom advantage over levothyroxine alone, large cohort data found no excess atrial fibrillation, heart failure or stroke, and the boxed warning exists because larger doses taken for weight loss can be life-threatening. (Source 2)

How do you get more of it?

It is prescription-only; there is no food or supplement form of T3. Under the heading "Adults" the label describes starting at 25 micrograms once daily, rising by 25 micrograms every one or two weeks if needed, to a usual maintenance of 25 to 75 micrograms, with a much lower 5 microgram start for older patients or those with heart disease. Children are dosed differently again: the label starts them at 5 micrograms once daily. Those figures record the label; they are not a recommendation. (Source 17)

If it is harmful, what reduces it?

Thyroid hormone is broken down by stripping iodine atoms off it, mainly in the liver, then conjugated and excreted via bile, gut and kidneys. Nothing speeds this up on demand; too much liothyronine is managed by lowering or stopping the dose, and the label’s adverse reactions are the symptoms of excess. (Source 1)

Why might someone be low in it or missing it?

Low T3 activity has real causes. The label lists hypothyroidism arising in the thyroid itself, in the pituitary or in the hypothalamus, congenital or acquired - these are what liothyronine replaces. Separately, drugs can lower T3: large doses of propranolol, dexamethasone at 4 mg a day or more, and amiodarone all reduce the conversion of T4 into T3. (Source 3)

Which whole foods contain it or feed it?

No whole food contains liothyronine the drug. The hormone itself occurs in animal thyroid tissue, which is what desiccated thyroid extract is made from - and a 2024 meta-analysis found desiccated thyroid raised total T3 and lowered T4 without changing heart rate, lipids or quality of life scores versus levothyroxine. Iodine is the raw material the thyroid needs, but eating iodine is not a route to taking T3. (Source 15)

What happens if you do not have it?

Without enough thyroid hormone from any source, metabolism slows and untreated hypothyroidism follows; replacement is the established treatment. Without liothyronine specifically, in someone already on levothyroxine, the pooled randomised evidence says symptoms and quality of life are no different - though more patients say they prefer having the T3. (Source 13)

How can you test for it?

Blood tests exist and are used, with a catch. In primary hypothyroidism TSH guides the dose, but the label says TSH is not a reliable measure of liothyronine adequacy in pituitary or hypothalamic hypothyroidism, where serum T3 should be used instead. A separate reliability problem is biotin: supplement doses interfere with the immunoassays and have produced falsely abnormal thyroid results and a wrong diagnosis. (Source 17)

References

1. DailyMed / US FDA Structured Product Label (PFIZER LABORATORIES DIV PFIZER INC, SPL effective 2026-05-18). CYTOMEL (liothyronine sodium) tablet - CLINICAL PHARMACOLOGY. 2026. Read the source
2. DailyMed / US FDA Structured Product Label (PFIZER LABORATORIES DIV PFIZER INC, SPL effective 2026-05-18). CYTOMEL (liothyronine sodium) tablet - FDA prescribing information (boxed warning section, LOINC 34066-1). 2026. Read the source
3. DailyMed / US FDA Structured Product Label (PFIZER LABORATORIES DIV PFIZER INC, SPL effective 2026-05-18). CYTOMEL (liothyronine sodium) tablet - INDICATIONS AND USAGE. 2026. Read the source
4. DailyMed / US FDA Structured Product Label (PFIZER LABORATORIES DIV PFIZER INC, SPL effective 2026-05-18). CYTOMEL (liothyronine sodium) tablet - DOSAGE AND ADMINISTRATION, thyroid suppression test and switching from levothyroxine. 2026. Read the source
5. Archives of General Psychiatry. Triiodothyronine augmentation in the treatment of refractory depression. A meta-analysis.. 1996. PMID 8792761, DOI 10.1001/archpsyc.1996.01830090090013. Read the source
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