Liothyronine (T3)
liothyronine sodium — Cytomel (oral), Triostat (IV)
Indications for Liothyronine
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Hypothyroidism (primary, secondary, tertiary — congenital or acquired) | Adults and paediatric patients, including neonates | Replacement or supplemental therapy | FDA Approved |
| Pituitary TSH suppression in thyroid cancer | Adults | Adjunct to surgery and radioiodine | FDA Approved |
| T3 suppression test (diagnostic aid) | Adults | Diagnostic: differentiating thyroid gland autonomy from normal function | FDA Approved |
| Myxedema coma / precoma (IV formulation) | Adults | Emergency IV replacement | FDA Approved |
Liothyronine is the synthetic form of the biologically active thyroid hormone triiodothyronine (T3). Unlike levothyroxine (T4), which must be converted to T3 peripherally, liothyronine acts directly. Its rapid onset and short half-life make it particularly useful during radioactive iodine (RAI) preparation for thyroid cancer, where a shorter withdrawal period (2 weeks versus 4–6 weeks for levothyroxine) minimises the duration of symptomatic hypothyroidism. However, the ATA 2014 guideline does not recommend liothyronine monotherapy as the standard of care for hypothyroidism due to serum T3 level fluctuations and the potential for more pronounced cardiovascular effects.
LT4/LT3 combination therapy for hypothyroidism: Some patients on levothyroxine monotherapy report persistent symptoms despite normalised TSH. Addition of low-dose liothyronine (5–10 mcg) with a corresponding reduction in levothyroxine has been studied, though the ATA 2014 guideline found no consistently strong evidence of superiority over levothyroxine monotherapy. (Evidence quality: Moderate — conflicting)
Augmentation of antidepressants in treatment-resistant depression: Liothyronine 25–50 mcg/day has been used to augment SSRIs and tricyclics, particularly in treatment-resistant major depressive disorder, supported by the STAR*D trial data. (Evidence quality: Moderate)
Thyroid hormone withdrawal protocol for RAI scanning: Given for 2–4 weeks after levothyroxine discontinuation, then stopped 2 weeks before RAI to allow TSH to rise while minimising hypothyroid symptoms. (Evidence quality: High)
Liothyronine Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Mild hypothyroidism — otherwise healthy adult | 25 mcg once daily | Increase by 25 mcg q1–2 weeks to 25–75 mcg/day | 75 mcg/day (usual) | Not first-line; levothyroxine is preferred for long-term replacement (ATA 2014) 25 mcg T3 ≈ 100 mcg T4 ≈ 1 grain desiccated thyroid in approximate equivalence |
| Severe / myxedema hypothyroidism — oral | 5 mcg once daily | Increase by 5–10 mcg q1–2 weeks; once 25 mcg/day reached, increase by 5–25 mcg q1–2 weeks. Usual maintenance 50–100 mcg/day | 100 mcg/day (usual ceiling) | Gradual titration critical to avoid cardiac events; slower escalation than mild hypothyroidism Higher maintenance than mild hypothyroidism reflects greater hormone deficit (FDA PI) |
| Elderly or known cardiac disease | 5 mcg once daily | Increase by 5 mcg q2 weeks | Individualised (minimise cardiovascular risk) | Rapid onset of T3 poses higher cardiac risk than levothyroxine If euthyroid state aggravates cardiac disease, reduce dose (FDA PI) |
| Thyroid cancer — RAI preparation withdrawal protocol | 25 mcg BID or TID (replace levothyroxine) | Continue for 2–4 weeks after stopping levothyroxine | 75 mcg/day typically | Stop liothyronine 2 weeks before RAI; allows TSH to rise >30 mIU/L for adequate RAI uptake Shorter withdrawal period (2 weeks) versus levothyroxine (4–6 weeks); minimises hypothyroid symptoms |
| T3 suppression test (diagnostic) | 75–100 mcg daily for 7 days | Single 7-day course | 100 mcg/day | Measure radioiodine uptake before and after; ≥50% suppression indicates normal thyroid-pituitary axis (FDA PI) |
| LT4/LT3 combination therapy (off-label adjunct) | 5–10 mcg once or twice daily | Titrate to normalise TSH; reduce levothyroxine by ~25–50 mcg per 5–10 mcg T3 added | Individualised; aim for physiologic T4:T3 ratio ~13:1 to 20:1 | Specialist-initiated only; limited evidence of benefit over LT4 monotherapy (ATA 2014) Consider divided dosing (BID) to minimise peak T3 fluctuations |
| Myxedema coma / precoma — IV | 25–50 mcg IV (initial); 10–20 mcg if cardiac disease | Subsequent doses q4–12h based on clinical response; target ≥65 mcg/day in initial days | Limited experience >100 mcg/day | ICU setting only; ≥65 mcg/day in initial days associated with lower mortality (Triostat PI) IV only; do not give IM or SC. Switch to oral when clinically stable. Give glucocorticoids concurrently |
Paediatric Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Congenital hypothyroidism — neonates | 5 mcg once daily | Increase by 5 mcg q3–4 days to response | ~20 mcg/day (infants a few months old) | Levothyroxine is strongly preferred for congenital hypothyroidism; liothyronine lacks long-term developmental data At 1 year, ~50 mcg/day may be needed; above 3 years, adult doses may be appropriate (FDA PI) |
When switching from levothyroxine to liothyronine, an approximate equivalence of 25 mcg liothyronine to 100 mcg levothyroxine is commonly used, though individual variability is substantial. Discontinue levothyroxine and initiate liothyronine at a low dose, titrating gradually. Residual effects from the long half-life of levothyroxine may persist for several weeks after switching, during which careful monitoring is essential. Liothyronine has a biologic potency approximately 4 times that of levothyroxine, so even small dose changes produce relatively large clinical effects.
Pharmacology of Liothyronine
Mechanism of Action
Liothyronine sodium is the synthetic sodium salt of L-triiodothyronine (T3), the most metabolically active thyroid hormone. Unlike levothyroxine, which serves as a prohormone requiring peripheral deiodination to T3, liothyronine directly binds to nuclear thyroid hormone receptors (TR-alpha and TR-beta) in target tissues. This receptor-DNA complex activates gene transcription and protein synthesis, producing the metabolic effects of thyroid hormone: increased basal metabolic rate, enhanced oxygen consumption, stimulation of carbohydrate and lipid metabolism, protein synthesis, and regulation of growth and differentiation. The rapid onset and shorter duration compared to levothyroxine are explained by T3’s lower affinity for serum binding proteins (particularly TBG), allowing faster tissue uptake but also more rapid clearance from the circulation. This produces pronounced peaks and troughs in serum T3 levels with once-daily dosing.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | ~95% absorbed orally within 4 hours (FDA PI). Tmax ~2–3 hours. Not significantly affected by food compared to levothyroxine. | Near-complete oral absorption makes liothyronine reliable for GI-compromised patients where levothyroxine absorption is impaired (e.g., coeliac disease, bariatric surgery). |
| Distribution | 99.5% protein bound (TBG, TBPA, albumin). Lower binding affinity to TBG than T4, resulting in a higher free fraction (~0.5% free vs ~0.03% for T4). | Lower protein binding explains faster tissue uptake, faster onset of action, and wider serum level fluctuations compared to levothyroxine. |
| Metabolism | Sequential deiodination in liver and peripheral tissues. Also undergoes glucuronidation, sulfation, and enterohepatic recirculation. | Hepatic enzyme inducers (phenytoin, carbamazepine, rifampicin) increase T3 clearance. Hepatic or renal impairment may alter metabolism. |
| Elimination | t½ ~1–2.5 days (FDA PI: ~2.5 days; PK studies: elimination phase ~23 h). Primarily renal (~80%) with ~20% faecal excretion. | Shorter half-life than T4 supports its use in the RAI withdrawal protocol (2-week stop versus 4–6 weeks for levothyroxine). However, it also means serum levels fluctuate more and steady-state is reached faster (~5–7 days). |
Side Effects of Liothyronine
As with levothyroxine, adverse effects of liothyronine are primarily those of iatrogenic hyperthyroidism resulting from overreplacement. Due to the rapid onset and peak-trough kinetics of T3, cardiovascular side effects are more pronounced and may occur even at therapeutic doses, particularly in elderly patients or those with pre-existing cardiac disease.
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Palpitations / tachycardia | ≥10% (more pronounced than with LT4) | Occurs rapidly due to direct T3 action on cardiac beta-receptors; peak effect within hours of dosing; dose-dependent |
| Tremor / nervousness / insomnia | ≥10% when TSH suppressed | May be transient at initiation; related to T3 peak levels rather than steady-state. Divided dosing (BID) may reduce |
| Heat intolerance / sweating | ≥10% | Classic hyperthyroid symptom; dose reduction required |
| Weight loss / increased appetite | ≥10% | Expected metabolic effect of excess T3; reverses with dose correction |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Headache | 1–10% | May occur with initiation or dose changes; usually self-limiting |
| Diarrhoea | 1–10% | Increased GI motility from excess thyroid hormone; dose-related |
| Menstrual irregularities | 1–5% | Both over- and underreplacement disrupt menstrual cycle; resolves with dose correction |
| Muscle weakness / cramps | 1–5% | More common during overreplacement; may reflect thyrotoxic myopathy |
| Skin rash | <1% | Rare; may be excipient-related; consider formulation switch |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Atrial fibrillation / arrhythmias | Higher risk than with LT4 due to T3 peak levels | Hours to days after initiation or dose increase | Reduce dose or withhold; cardiology evaluation; ECG monitoring |
| Angina pectoris / myocardial infarction | Rare; elevated risk in patients with coronary artery disease | Hours to days | Withhold liothyronine; stabilise cardiac status; restart at lower dose or switch to levothyroxine |
| Accelerated bone loss | Risk with chronic supraphysiologic dosing | Months to years | Use lowest effective dose; DEXA monitoring in postmenopausal women; calcium and vitamin D supplementation |
| Acute adrenal crisis | Rare; in patients with undiagnosed adrenal insufficiency | Days after initiation | Treat AI with glucocorticoids BEFORE starting liothyronine; contraindicated in uncorrected AI |
The rapid onset and direct cardiac effects of liothyronine (T3 peaks within 2–3 hours of dosing) create a higher risk of cardiac events compared to levothyroxine. In patients with pre-existing coronary artery disease or the elderly, start at the lowest dose (5 mcg/day) and titrate slowly. If cardiac symptoms develop at any dose, reduce or discontinue liothyronine and consider switching to levothyroxine, which provides more stable hormone levels.
Drug Interactions with Liothyronine
Liothyronine shares the same pharmacodynamic interaction profile as levothyroxine. However, because T3 is less affected by GI absorption interference (95% absorption versus 40–80% for T4), interactions that reduce levothyroxine efficacy through impaired absorption (calcium, iron, PPIs) are less clinically significant with liothyronine. Interactions affecting hepatic metabolism, protein binding, and anticoagulant response remain important.
Monitoring for Liothyronine
- Serum TSH6–8 weeks after initiation or dose change; q6–12 months when stable
RoutinePrimary monitoring parameter for primary hypothyroidism. TSH is NOT reliable for secondary/tertiary hypothyroidism or during the immediate hours after dosing (T3 peaks suppress TSH transiently). Draw blood before the morning dose for consistency. - Serum Total T3With TSH at each assessment
RoutineMore appropriate than TSH for assessing immediate response. Draw trough level (before morning dose). In secondary/tertiary hypothyroidism, T3 is the primary target for dose adequacy (FDA PI). - Cardiac StatusBaseline ECG in elderly/cardiac patients; symptoms at every visit
Trigger-basedMonitor for palpitations, chest pain, dyspnoea, and new arrhythmias. T3 has more pronounced cardiac effects than T4 due to direct beta-receptor stimulation. Continuous cardiac monitoring for IV liothyronine in myxedema coma. - Bone DensityBaseline and periodic in postmenopausal women on chronic therapy
Trigger-basedChronic supraphysiologic thyroid hormone accelerates bone turnover. Use the lowest effective dose and monitor with DEXA. - Paediatric GrowthEvery visit in children
RoutineHeight, weight, developmental milestones, and bone age. To minimise hyperactivity risk, start at one-fourth the full replacement dose and increase weekly by one-fourth increments (FDA PI). - INR (if on warfarin)Closely during initiation and dose changes
Trigger-basedThyroid hormone status changes alter warfarin requirements; check INR frequently until thyroid status is stable. - Blood Glucose (diabetics)Increased frequency during thyroid status changes
Trigger-basedCorrection of hypothyroidism increases metabolic rate and may worsen glycaemic control; adjust antidiabetic therapy accordingly.
Contraindications & Cautions
Absolute Contraindications
- Uncorrected adrenal cortical insufficiency — thyroid hormone increases cortisol clearance and can precipitate adrenal crisis; treat with glucocorticoids first (FDA PI)
- Untreated thyrotoxicosis — exogenous T3 will worsen hyperthyroid state
- Known hypersensitivity to liothyronine sodium or any excipient
Relative Contraindications (Specialist Input Recommended)
- Acute myocardial infarction — T3’s rapid cardiac effects pose a high risk; withhold unless clinically essential
- Severe angina pectoris or uncontrolled arrhythmias — if required, start at 5 mcg/day with close cardiac monitoring (FDA PI)
Use with Caution
- Elderly patients — start 5 mcg/day; increase by 5 mcg q2 weeks; greater likelihood of occult cardiac disease
- Cardiovascular disease — rapid onset of T3 increases myocardial oxygen demand more acutely than T4
- Diabetes mellitus — correction of hypothyroidism may worsen glycaemic control
- Suspected thyroid gland autonomy — exogenous T3 effects are additive to endogenous production (FDA PI)
- Pregnancy — do not discontinue; dose adjustment required. Levothyroxine is generally preferred in pregnancy
Thyroid hormones, including liothyronine, 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. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in combination with sympathomimetic amines such as those used for their anorectic effects.
Patient Counselling
Purpose of Therapy
Liothyronine replaces a thyroid hormone (T3) that your body normally produces. It is usually prescribed for specific situations where a rapid-acting thyroid hormone is needed, such as preparation for radioactive iodine treatment or when levothyroxine alone is not adequately controlling symptoms. In most cases, levothyroxine is the preferred long-term medication for hypothyroidism, and your prescriber will advise you on the appropriate duration of liothyronine use.
How to Take
Take liothyronine tablets by mouth once daily, or as directed by your prescriber (some patients may take it in divided doses). Liothyronine can be taken with or without food, though consistency in timing is recommended. Do not stop taking this medication without medical advice. If you are scheduled for radioactive iodine treatment, your prescriber will instruct you when to stop liothyronine to allow your TSH to rise.
Sources
- Pfizer Inc. Cytomel (liothyronine sodium) tablets — Full Prescribing Information. Revised 2018. FDA Label: CytomelPrimary US prescribing reference for oral liothyronine; source for dosing by clinical scenario, PK parameters, and contraindications.
- XGen Pharmaceuticals DJB, Inc. Liothyronine Sodium Injection — Prescribing Information. DailyMed: TriostatPI for IV liothyronine for myxedema coma/precoma; source for IV dosing, onset of action, and administration route restrictions.
- DailyMed. Cytomel — liothyronine sodium tablet. National Library of Medicine. DailyMed: CytomelNLM-hosted structured label used to cross-verify dosing, adverse effects, and drug interaction data.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751. https://doi.org/10.1089/thy.2014.0028Landmark ATA guideline establishing levothyroxine monotherapy as standard; provides evidence review on LT4/LT3 combination therapy and liothyronine monotherapy.
- Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association management guidelines for adult patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26(1):1–133. https://doi.org/10.1089/thy.2015.0020ATA thyroid cancer guideline; includes protocols for thyroid hormone withdrawal using liothyronine before RAI therapy.
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the AACE and ATA. Endocr Pract. 2012;18(6):988–1028. https://doi.org/10.4158/EP12280.GLAACE/ATA consensus guideline on hypothyroidism; provides context for T3 use in clinical practice and discusses dose equivalence.
- Nierenberg AA, Fava M, Trivedi MH, et al. A comparison of lithium and T3 augmentation following two failed medication treatments for depression: a STAR*D report. Am J Psychiatry. 2006;163(9):1519–1530. https://doi.org/10.1176/ajp.2006.163.9.1519STAR*D Level 3 trial comparing liothyronine (T3) augmentation with lithium for treatment-resistant depression; supports T3 augmentation as a tolerable strategy.
- Wiersinga WM, Duntas L, Fadeyev V, Nygaard B, Vanderpump MPJ. 2012 ETA guidelines: the use of L-T4 + L-T3 in the treatment of hypothyroidism. Eur Thyroid J. 2012;1(2):55–71. https://doi.org/10.1159/000339444European Thyroid Association guideline reviewing evidence for LT4/LT3 combination therapy; provides recommendations on candidate selection and dosing ratios.
- Saravanan P, Chau WF, Roberts N, Vedhara K, Greenwood R, Dayan CM. Psychological well-being in patients on ‘adequate’ doses of L-thyroxine: results of a large, controlled community-based questionnaire study. Clin Endocrinol (Oxf). 2002;57(5):577–585. https://doi.org/10.1046/j.1365-2265.2002.01654.xFoundational study documenting persistent symptoms in some levothyroxine-treated patients, prompting interest in T3-containing regimens.
- Jonklaas J, Burman KD, Wang H, Latham KR. Single dose T3 administration: kinetics and effects on biochemical and physiological parameters. Thyroid. 2015;25(11):1209–1216. https://doi.org/10.1089/thy.2015.0318PK study documenting Tmax ~2.5 hours, half-life ~22 hours, and sustained TSH suppression beyond 24 hours after a single 50 mcg liothyronine dose.
- Celi FS, Zemskova M, Engel AD, et al. Pharmacokinetics of L-triiodothyronine in patients undergoing thyroid hormone therapy withdrawal. Thyroid. 2019;29(10):1475–1483. https://doi.org/10.1089/thy.2019.0101PK study in athyreotic patients; establishes two-compartment model with distribution t½ ~2.3 h and elimination t½ ~23 h; informs BID dosing strategies.
- Bianco AC, Salvatore D, Gereben B, Berry MJ, Larsen PR. Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocr Rev. 2002;23(1):38–89. https://doi.org/10.1210/edrv.23.1.0455Comprehensive review of deiodinase biology; explains why T3 from peripheral T4 conversion provides more stable levels than exogenous T3 administration.