Liothyronine

Brand names: Cytomel. Also called T3, LT3, triiodothyronine, L-triiodothyronine.

Thyroid hormone (synthetic T3) 1 graded human study Last checked Oct 23, 2026

The short answer

From 6 sources

Liothyronine is a synthetic copy of T3, the active thyroid hormone that tissues normally make from T4. It works within hours and wears off faster than levothyroxine. Its label covers hypothyroidism, TSH suppression in thyroid cancer and a diagnostic suppression test. Adding it to levothyroxine has not improved symptoms in trials so far.

Key facts

Form: Oral tablet¹
Labelled for: Hypothyroidism, thyroid cancer TSH suppression, suppression test¹
Therapeutic index: Narrow, per the label: doses are titrated carefully and the response monitored¹
Adult start: 25 mcg once daily (5 mcg if older or heart disease)¹
Onset: Within hours; peak response in 2 to 3 days¹
Standard of care?: No: levothyroxine alone remains standard³
Added T3 in trials: No symptom gain across 11 trials⁴

Which liothyronine products are approved, and for what?

Liothyronine tablets (Cytomel and generics) are FDA-approved prescription drugs for hypothyroidism, TSH suppression in thyroid cancer and the thyroid suppression test. Like every thyroid hormone label, Cytomel carries a boxed warning that thyroid hormone must not be used to treat obesity or for weight loss.

Status by form

1 form

Liothyronine products and their regulatory status
Form	Status	Approved for	Boxed warning
Tablets 5, 25 and 50 mcgCytomel, generics	FDA-approved prescription drug^1,2	Hypothyroidism (primary, secondary, tertiary); TSH suppression in well-differentiated thyroid cancer; thyroid suppression test	Yes: not for treatment of obesity or for weight loss; larger doses can cause serious or life-threatening toxicity

What is liothyronine and how does it work?

Most of the body's active T3, about 80%, normally comes from T4 converted in tissues. Liothyronine supplies T3 directly: it is almost fully absorbed within four hours, starts acting within a few hours and peaks in two to three days. It binds less tightly to blood proteins than T4, so it clears faster.

How it works

Thyroid hormone (synthetic T3)

T3 enters the cell nucleus and binds thyroid hormone receptors on DNA, switching on the genes behind metabolic activity. Normally about 80% of T3 is made from T4 in tissues; liothyronine supplies it directly. It is about 95% absorbed within four hours and binds blood proteins less tightly than T4, so it acts and clears faster.¹

What doses are on the label and in studies?

For hypothyroidism the label starts adults at 25 mcg once daily and raises it by 25 mcg every one to two weeks, to a usual 25 to 75 mcg. Older adults and people with heart disease start at 5 mcg. The thyroid suppression test uses 75 to 100 mcg daily for seven days.

Label and study amounts, not a dose for you. A prescriber decides whether and how much; these show what the labels print and what trials used.

Doses by use

4 rows

Liothyronine doses as labelled or studied
Use	Form	Amount and schedule	Basis
Hypothyroidism, adults	Tablets	Start 25 mcg once daily; increase by 25 mcg every 1 or 2 weeks; usual maintenance 25 to 75 mcg once daily¹	Label
Hypothyroidism, older adults or heart disease	Tablets	Start 5 mcg once daily; increase by 5 mcg at the same intervals¹	Label
Thyroid suppression test	Tablets	75 to 100 mcg daily for 7 days, with radioiodine uptake measured before and after; 50% or more suppression indicates a normal axis¹	Label
Switching from levothyroxine	Tablets	Stop levothyroxine and start liothyronine at a low dose, rising gradually by response¹	Label

What is liothyronine combined with?

Liothyronine is sometimes added to levothyroxine for people who still feel unwell. A meta-analysis of 11 trials found no gain in symptoms, mood or quality of life, and the American Thyroid Association keeps levothyroxine alone as the standard. A 2021 international consensus agreed there is equipoise for better-designed trials.

Liothyronine added to levothyroxine

For people who feel unwell despite normal tests on levothyroxine.

A meta-analysis of 11 randomized trials found combination therapy no better than levothyroxine alone for pain, depression, anxiety, fatigue, quality of life, weight or lipids. The 2014 American Thyroid Association guideline found no consistently strong evidence for combination therapy; a 2021 ATA, BTA and ETA consensus agreed there is equipoise for new trials and set out how they should be designed.^4,3,5

Checked: TSH, T3 and symptoms.

How does liothyronine relate to fertility and pregnancy?

As with levothyroxine, untreated hypothyroidism carries risks in pregnancy, and the label says not to stop liothyronine but to monitor TSH and adjust the dose. Experience in pregnancy has not shown more birth defects or miscarriages. Overtreatment can cause menstrual irregularities.

Fertility, pregnancy and breastfeeding

Pregnancy: The label says not to stop liothyronine during pregnancy, to monitor TSH and adjust the dose; experience has not shown more birth defects or miscarriages.¹
Menstrual changes: Menstrual irregularities are listed among signs of overtreatment.¹

What gets checked during treatment?

TSH and T3 levels plus symptoms guide the dose. With pituitary or hypothalamic hypothyroidism, TSH is not a reliable guide and the label uses serum T3 instead. When switching from levothyroxine, the old drug's effects can linger for weeks, so liothyronine starts low and rises slowly.

Monitoring

3 checks

What is monitored on liothyronine
Check	When	Why
TSH and T3	Periodically, and after each dose change¹	To keep thyroid levels in range; symptoms are weighed too
Serum T3 instead of TSH	Pituitary or hypothalamic (secondary or tertiary) hypothyroidism¹	TSH is not a reliable guide in these conditions
Heart rhythm and symptoms	Especially in older adults and heart disease¹	Atrial fibrillation and other cardiac reactions with overtreatment

What does the research show?

Trials of combination T4 plus T3 have not shown that it beats levothyroxine alone: a meta-analysis of 11 trials with 1,216 patients found no difference in pain, depression, anxiety, fatigue, quality of life, weight or lipids. The 2014 American Thyroid Association guideline reached the same conclusion.

Every graded row

1 rows from 1 studies

Grade BSymptoms with added T3 vs levothyroxine aloneCombination therapyNo effectMeta-analysis · 11 studies
n 1,216 · 2006

No difference in pain, depression, anxiety, fatigue, quality of life, weight or lipids; adverse events similar.

Design: Meta-analysis, 11 studies, n = 1,216, 2006
Population: Adults with overt hypothyroidism in 11 randomized trials
Form: T4 plus T3 vs T4 alone
Dose: Various
Duration: Various
Source: Reference 4 · PubMed 16670166 (opens PubMed)

What are the side effects and risks?

Side effects come mainly from too high a dose: arrhythmias, chest pain, breathlessness, headache, nervousness, insomnia, tremor, heat intolerance, weight loss, diarrhea and menstrual changes. Older adults and people with heart disease start at 5 mcg because of atrial fibrillation risk. Uncorrected adrenal insufficiency is the one contraindication.

Regulatory status. Liothyronine tablets are FDA-approved prescription drugs with a boxed warning against use for obesity or weight loss.^1,2

Most common side effects on the label

Arrhythmias and palpitations¹
Headache, nervousness, irritability, insomnia¹
Tremor and muscle weakness¹
Heat intolerance, increased appetite, weight loss¹
Diarrhea¹
Menstrual irregularities¹

Warnings on the label

Narrow therapeutic index: The Cytomel label says liothyronine has a narrow therapeutic index: over- or undertreatment can affect growth, heart function, bone, reproductive and cognitive function, and glucose and lipid metabolism, so the dose is titrated carefully and the response monitored.¹
Overtreatment: Too high a dose causes hyperthyroidism, including arrhythmias and myocardial infarction; replacement-range doses do not cause weight loss in people with normal thyroid function, and larger doses can be seriously toxic.¹
Heart disease and older age: Starting below the full dose is advised because of atrial fibrillation and other cardiac reactions.¹
Adrenal crisis: With untreated adrenal insufficiency, glucocorticoids are replaced before starting.¹
Diabetes: Glycemic control can worsen and medicine needs rise.¹

Who should not use it (label contraindications)

Uncorrected adrenal insufficiency.¹

What does liothyronine interact with?

Bile acid sequestrants such as cholestyramine and colesevelam, and ion exchange resins such as sevelamer, reduce absorption, so the label gives liothyronine at least 4 hours before them or monitors TSH. Like levothyroxine, thyroid hormone can change warfarin and diabetes medicine needs. Combined with stimulant appetite suppressants, larger doses can be dangerous.

Interactions

4 agents

Published interactions with liothyronine
With	What happens	Evidence
Bile acid sequestrants and ion exchange resinsCholestyramine, colesevelam, colestipol, sevelamer	Reduce absorption; liothyronine is taken at least 4 hours before them, or TSH is monitored.¹	Drug label
Oral anticoagulantsWarfarin	Liothyronine increases the anticoagulant effect; coagulation tests are watched when the dose rises or hypothyroidism is corrected.¹	Drug label
Sympathomimetic appetite suppressantsStimulant diet drugs	With larger thyroid hormone doses, serious or life-threatening toxicity.¹	Drug label
Diabetes medicinesInsulin, oral agents	Glycemic control can worsen and needs can rise.¹	Drug label

Check your own medicines and supplements in the Interaction map: every edge is sourced, and nothing you enter leaves your browser.

How do the options compare?

Liothyronine and levothyroxine

Liothyronine and levothyroxine
	Liothyronine (T3)	Levothyroxine (T4)
What it is	The active hormone itself	The thyroid's main hormone, converted to T3 in tissues
Usual adult replacement	25 to 75 mcg daily	About 1.6 mcg/kg/day
Dose steps	25 mcg every 1 to 2 weeks	12.5 to 25 mcg every 4 to 6 weeks
Onset	Within hours; peak in 2 to 3 days	Peak effect may take 4 to 6 weeks

Frequently asked questions

Common questions

5 answers

Is liothyronine better than levothyroxine?

Not as a rule. Levothyroxine alone remains the standard of care, and trials adding T3 found no improvement in symptoms or quality of life.^3,4

How fast does liothyronine work?

The label says it starts acting within a few hours and reaches its maximum effect in two to three days, much faster than levothyroxine.¹

Why is TSH not used to dose liothyronine in pituitary hypothyroidism?

When the pituitary is the problem, TSH does not reflect thyroid hormone status, so the label uses serum T3 to judge the dose instead.¹

What is the thyroid suppression test?

Liothyronine 75 to 100 mcg daily for seven days, with radioiodine uptake measured before and after; a drop of 50% or more indicates a normal thyroid-pituitary axis.¹

Can liothyronine help with weight loss?

No. Its label, like every thyroid hormone label, warns that it is ineffective for weight loss at replacement doses and can be seriously toxic at higher ones.¹

Sources

Sources cited on this page

6 sources

1Cytomel (liothyronine sodium) tablets: prescribing information (DailyMed, version effective 2026-05-18)Label Pfizer, 2026
2Liothyronine sodium tablets: prescribing information (DailyMed, version effective 2020-07-01)Label Teva, 2020
3Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacementGuideline Thyroid, 2014 · PMID 25266247
4Thyroxine-triiodothyronine combination therapy versus thyroxine monotherapy for clinical hypothyroidism: meta-analysis of randomized controlled trialsMeta-analysis J Clin Endocrinol Metab, 2006 · PMID 16670166
5Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus DocumentGuideline Thyroid, 2021 · PMID 33276704
6Synthroid (levothyroxine sodium) tablets: prescribing information (DailyMed, version effective 2024-02-20)Label AbbVie, 2024

What changed

Change log

1 entry

Dated changes to this page
Date	Change	Who and why
Oct 23, 2026	Page created: label status, doses for hypothyroidism and the suppression test, switching from levothyroxine, the combination-therapy evidence, monitoring, safety and absorption interactions.	Writer: Kymata Health EditorialTrigger: Owner request (2026-10-22): hormone library

Kymata Health hormone library. Educational information only; it is not medical advice and does not replace a clinician. Doses are shown as labels and studies print them; a prescriber decides whether and how any hormone is used.