Metabolic Thyroid Hormone (active)FDA-approved medicinePrescription drug

T3 (Liothyronine / Cytomel)

T3 · Cytomel · Liothyronine Sodium

Evidence: Approved label. Example dose: finalize with a provider. Not a fact-check stamp.

12.5mcg – 12.5-25 mcg Oral 1-3x daily

Synthetic triiodothyronineFDA-ApprovedWADA Not Prohibited (monitored)

Liothyronine sodium (T3) is the synthetic form of the active thyroid hormone 3,5,3'-triiodothyronine. It is the biologically active form produced endogenously both by the thyroid gland directly and by peripheral deiodination of T4 (thyroxine) in the liver, kidneys, and other tissues. FDA-approved under the brand names Cytomel and Triostat, it is used for the treatment of hypothyroidism, myxedema coma (intravenous), and as a diagnostic suppression test for thyroid cancer.

Quick Start

Route

Oral

Start low

12.5mcg Oral

Frequency

1-3x daily

Timing

Take on an empty stomach, 30-60 minutes before food

Given the ~24 hour half-life, once-daily morning dosing is standard for replacement therapy. For higher off-label doses, splitting AM/PM smooths cardiac load. Always titrate up by 12.5 mcg increments every 3-5 days, monitoring resting heart rate, blood pressure, and symptoms (tremor, heat intolerance, anxiety). Taper down symmetrically at cycle end to avoid rebound hypothyroidism.

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Start here. Every protocol below is ordered lowest-first. Begin at the smallest effective dose, hold about a week to assess tolerance, and step up only if needed.

Tiered Protocols · lowest first

Conservative start

12.5mcg · Oral · 1-3x daily

conservative starter

12.5mcg

1-3x daily

Expert

Metabolic Enhancement

12.5-25 mcg · Oral · 2x Daily

human clinical trial · Celi et al. 2011 (J Clin Endocrinol Metab)

Direct, active thyroid hormone. Short half-life necessitates splitting the dose. Will rapidly suppress endogenous thyroid output.

12.5-25 mcg

2x Daily

◆

How it works

Liothyronine is identical to endogenous T3 and acts via nuclear thyroid hormone receptors (TR-alpha and TR-beta). Binding recruits co-activators and activates transcription of genes that regulate basal metabolic rate, oxygen consumption, protein turnover, carbohydrate and lipid metabolism, and heat production. T3 drives mitochondrial biogenesis and increases Na+/K+-ATPase activity, raising cellular energy expenditure.

Compared to T4, T3 has roughly 4x the receptor-level potency and does not require peripheral deiodination, so its onset is within hours rather than days. The same property makes overshoot easier - a dose change takes effect quickly, and the user feels the effects (both desired and adverse) sooner.

Exogenous T3 suppresses endogenous thyroid production via negative feedback on the HPT axis. On stopping, there is a lag of several days to weeks before endogenous production resumes normal levels, which is why tapered discontinuation is standard. At bodybuilding doses (50-100 mcg/day), cardiac workload rises meaningfully and muscle protein catabolism can offset the desired lean-mass preservation if insufficient caloric and protein intake is provided.

What to expect

Early

Days 1–7

First 24-72 hours: warming sensation, mild tachycardia, slight appetite reduction. Effects noticeable within hours of first dose, unlike T4 which takes days.

Mid

Weeks 2–4

Week 1-3: measurable fat loss under caloric deficit, elevated basal body temperature, reduced cold tolerance. Cardiac load is palpable - monitor resting heart rate daily.

Later

Weeks 4–12

Week 3-5: peak metabolic acceleration. Extending beyond 5-6 weeks of supraphysiologic dosing without a break compounds cardiac and catabolic risk. Taper down symmetrically to avoid rebound hypothyroid phase.

Evidence

HumanStrong

AnimalPresent

In-vitroPresent

Each reference below was re-fetched from its PubMed record, and the quoted sentence was confirmed to appear in that abstract and to support the claim it sits under. Follow any of them to read the source.

Exact-molecule evidence boundary: the live abstract reports the quoted finding for T3 (Liothyronine / Cytomel); no broader inference is made.

Despite more than 20 years of debate, the use of liothyronine for this indication remains controversial, as numerous randomised trials have failed to show a benefit of treatment regimens that combine liothyronine (T3) with levothyroxine over levothyroxine monotherapy.

Use of liothyronine (T3) in hypothyroidism: Joint British Thyroid Association/Society for endocrinology consensus statement. Clinical endocrinology · 2023 · PMID 37272400

Evidence scope. Exact molecule wording appears in the quoted live sentence. Only the population/model, formulation, route, and outcome expressly stated there are supported.

Claims we could not support

No abstract we checked supports these for this molecule at this route. That is not the same as saying they are false — it marks where the evidence is missing.

Dose/route: 12.5mcg; Oral; 1-3x daily. No selected live abstract sentence verified this complete regimen.

Mechanism as written in source JSON. No selected live abstract sentence verified the complete mechanism at the card formulation/route and relevant population/model.

Primary safety claim as written in source JSON. No selected live abstract sentence verified the complete safety claim for this exact formulation and route.

Side effects & safety

Commonly reported

Elevated resting heart rate / tachycardia
Heat intolerance, increased sweating
Mild tremor, anxiety, jitteriness
Sleep disturbance (especially if dosed late in the day)
Increased appetite or transient nausea
Muscle protein catabolism at higher doses (potential strength loss if diet is under-supportive)

Less common & notes

Palpitations, atrial fibrillation (especially in older users or those with underlying cardiac disease), chest pain, hypertension, accelerated bone turnover (osteoporosis risk with chronic supraphysiologic use), menstrual irregularities, hair thinning or hair texture changes.
Rare but serious: thyroid storm, myocardial infarction at very high doses.
Abrupt discontinuation from high doses produces a period of functional hypothyroidism (fatigue, weight gain, cold intolerance) lasting 2-6 weeks until endogenous production recovers.

Pharmacokinetics

Illustrative plasma concentration · 4 d

Half-life

~24 hours

~1 day

Peak · Tmax

2-4 hours

Elimination

5-7 days

Bioavailability

85-95%

Oral (empty stomach)

Duration of action

18-30 hours

Per dose - supports once daily or split AM/PM dosing

Clearance

Hepatic conjugation and deiodination. Renal and biliary excretion of metabolites.

Storage. Store at controlled room temperature (20-25°C / 68-77°F). Protect from light and moisture - liothyronine is moisture-sensitive. Keep in original container with desiccant where provided. Shelf life typically 2-3 years from manufacture date. Keep out of reach of children - small doses have outsized effect in children.

Regulatory status

FDA-approved under brand names Cytomel (Pfizer / King Pharmaceuticals) and Triostat (IV formulation). Prescription-only in the United States. Not a controlled substance. Not prohibited by WADA - thyroid hormones are not on the current WADA prohibited list, though they are monitored and some sporting bodies impose therapeutic use exemption requirements. Internationally available under various brand names including Thybon, Cynomel, and Tertroxin.

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