Liothyronine (T3)

FDA Approved

Thyroid Hormone | Metabolic Optimization

Weight: 650.97 Da

Half-life: ~1 day

5 studies

2023 latest

2 recent

FDA Approved

Dose 25-75 mcg/day (replacement)

Frequency 1-2x daily (oral)

Cycle 6-8 weeks (fat loss) or ongoing (replacement)

Storage Room temperature (68-77F). Protect from light and moisture.

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Liothyronine (T3) is the biologically active form of thyroid hormone, responsible for regulating basal metabolic rate, thermogenesis, protein synthesis, and cellular energy metabolism throughout the body. Unlike levothyroxine (T4), which serves primarily as a prohormone requiring peripheral conversion by deiodinase enzymes, T3 acts directly on nuclear thyroid hormone receptors to exert its metabolic effects. It has been FDA-approved since the 1950s for the treatment of hypothyroidism, myxedema coma, and as a diagnostic agent in thyroid suppression tests. Pharmaceutical T3 is available as Cytomel (brand) and generic liothyronine sodium tablets. In performance and biohacking contexts, T3 is widely used during cutting phases to accelerate fat loss by directly upregulating metabolic rate, and by individuals seeking metabolic optimization when peripheral T4-to-T3 conversion is impaired due to caloric restriction, stress, or other factors.

Mechanism of Action

Liothyronine enters target cells via monocarboxylate transporter 8 (MCT8) and other thyroid hormone transporters, then binds to nuclear thyroid hormone receptors (primarily TR-beta in metabolic tissues and TR-alpha in cardiac tissue). The T3-receptor complex forms heterodimers with retinoid X receptors (RXR) and binds to thyroid hormone response elements (TREs) on DNA, directly modulating gene transcription. This increases expression of Na+/K+ ATPase (driving cellular energy expenditure), uncoupling proteins (UCP1 in brown adipose tissue, promoting thermogenesis), and enzymes involved in fatty acid oxidation, gluconeogenesis, and protein turnover. T3 also upregulates beta-adrenergic receptor density, which amplifies catecholamine sensitivity and contributes to increased heart rate, lipolysis, and thermogenesis. At physiological doses, T3 supports healthy metabolism; at supraphysiological doses, it shifts the body into a catabolic state where both fat and lean tissue can be catabolized for energy.

01 Direct and rapid increase in basal metabolic rate

02 Enhanced fat oxidation and lipolysis

03 Improved energy levels and reduction of hypothyroid fatigue

04 Faster-acting than T4 with effects noticeable within hours to days

05 Useful when peripheral T4-to-T3 conversion is impaired (illness, caloric deficit, stress)

06 Supports body temperature regulation and thermogenesis

07 Can improve cognitive clarity and reduce brain fog associated with low thyroid function

Molecular Data

Molecular Weight

650.97 Da

Type

Thyroid hormone (C15H12I3NO4)

Peak 0.0 mcg

Trough 0.0 mcg

SS Peak 0.0 mcg

SS Trough 0.0 mcg

Research Indications

Endocrine / Thyroid

Hypothyroidism most effective

Treatment of overt and subclinical hypothyroidism, either as monotherapy or in combination with levothyroxine (T4). Particularly useful in patients who report persistent symptoms despite normalized TSH on T4 monotherapy, suggesting poor peripheral conversion.

Myxedema Coma most effective

Emergency treatment of severe hypothyroidism presenting as myxedema coma. Intravenous T3 is preferred due to its rapid onset of action compared to T4.

T4-to-T3 Conversion Deficit effective

Supplementation in individuals with documented poor conversion of T4 to T3 due to deiodinase polymorphisms (DIO2 gene variants), chronic caloric restriction, illness, or elevated reverse T3 levels.

Thyroid Suppression Testing most effective

Diagnostic use in thyroid suppression tests to differentiate between types of thyroid autonomy.

Body Composition / Performance

Fat Loss / Cutting effective

Widely used in bodybuilding and physique sports to accelerate fat loss during caloric deficit. T3 increases basal metabolic rate by 10-15% at moderate doses, promoting faster utilization of stored fat. Most effective when combined with adequate protein intake and resistance training to minimize muscle catabolism.

Metabolic Rate Optimization moderate

Used by biohackers and longevity-focused individuals to maintain optimal metabolic rate, particularly during extended caloric restriction, reverse dieting, or in the context of metabolic adaptation (adaptive thermogenesis).

Contest Preparation effective

Standard component of bodybuilding contest prep stacks, typically combined with anabolic steroids (to offset catabolism) and beta-2 agonists for maximal fat mobilization in the final 6-8 weeks before competition.

Dosing Protocols

Liothyronine is administered orally as tablets, which is the standard and essentially only route used outside of emergency clinical settings. Oral bioavailability is approximately 95%, making it highly effective by mouth. Tablets are available in 5 mcg, 25 mcg, and 50 mcg strengths. The short half-life (~24 hours) means that split dosing throughout the day can provide more stable blood levels, though once-daily dosing is common for replacement therapy.

Goal	Dose	Frequency	Route
Thyroid Replacement - Standard	25-75 mcg/day	Once daily or split into 2-3 doses	Oral
Thyroid Replacement - Conservative Start	5-25 mcg/day	Once daily	Oral
Fat Loss - Moderate	25-50 mcg/day	Split into 2 doses (AM and early PM)	Oral
Fat Loss - Aggressive (Contest Prep)	50-75 mcg/day	Split into 2-3 doses	Oral
T4/T3 Combination Therapy	5-15 mcg/day (added to T4)	Once daily or split into 2 doses	Oral

Interactions

Clenbuterol

Synergistic for fat loss as T3 increases metabolic rate while clenbuterol stimulates beta-2 adrenergic lipolysis. However, the combination carries significant cardiac risk: both compounds increase heart rate, and T3 upregulates beta-adrenergic receptor density, amplifying clenbuterol's cardiovascular effects. Monitor heart rate and blood pressure closely. Start doses conservatively and do not escalate both simultaneously.

monitor

Testosterone

Commonly stacked during cutting phases. Testosterone helps preserve lean muscle mass that T3 would otherwise catabolize at higher doses. The combination allows for more aggressive T3 dosing with reduced risk of muscle wasting. No direct pharmacological conflict.

compatible

Growth Hormone

GH and T3 have complementary metabolic effects. GH promotes lipolysis and directs freed fatty acids toward oxidation, while T3 increases overall metabolic rate and energy expenditure. GH also enhances peripheral T4-to-T3 conversion, and exogenous GH use can deplete T3 levels over time, making concurrent T3 supplementation particularly logical during GH protocols.

synergistic

What to Expect

Hours 2-6

Initial onset of action after first dose. Subtle increase in heart rate, body temperature, and energy. T3 is absorbed rapidly with peak serum levels reached within 2-4 hours of oral administration.

Day 1-3

Noticeable increase in energy, warmth, and metabolic activity. Some individuals report improved mental clarity and reduced brain fog. Mild appetite increase is common as metabolism accelerates. Heart rate may be slightly elevated.

Week 1-2

Steady-state blood levels achieved. Measurable increase in basal metabolic rate. Visible changes in body composition begin, particularly in individuals already in a caloric deficit. TSH suppression becomes apparent on lab work. Some sweating and heat intolerance at higher doses.

Week 3-6

Peak fat loss effects. Significant reduction in body fat when combined with proper diet and training. At doses above 50 mcg/day, risk of lean tissue catabolism increases without concurrent anabolic support. Endogenous thyroid production is suppressed.

Week 6-8+

Diminishing returns at extended durations. Risk of muscle wasting, bone density reduction, and cardiac stress increases with prolonged use. Taper protocol should begin. Endogenous thyroid function typically recovers within 2-6 weeks after discontinuation, though prolonged high-dose use may extend recovery.

Side Effects & Safety

Common Side Effects

Tachycardia and palpitations (increased heart rate, especially at higher doses)
Anxiety, nervousness, and irritability
Insomnia and disrupted sleep architecture
Increased sweating and heat intolerance
Tremor (fine hand tremor, similar to hyperthyroid presentation)
Increased appetite despite accelerated metabolism
Loose stools or increased bowel frequency

Stop Signs - Discontinue if:

Chest pain, pressure, or significant palpitations
Resting heart rate consistently above 100-110 bpm
Severe anxiety or panic attacks
Significant unexplained muscle weakness or wasting
Signs of adrenal crisis: severe fatigue, hypotension, nausea, dizziness
Irregular heartbeat or sensation of skipped beats

Contraindications

Untreated adrenal insufficiency (must correct cortisol deficiency before starting T3)
Acute myocardial infarction or unstable angina
Thyrotoxicosis or untreated hyperthyroidism
Known hypersensitivity to liothyronine or any tablet excipients
Uncorrected adrenal cortical insufficiency (risk of adrenal crisis)

Quality Checklist

Good Signs

Pharmaceutical-grade tablets from licensed manufacturer (Cytomel or reputable generic)
Proper labeling with strength (5, 25, or 50 mcg), lot number, and expiration date
Tablets are uniform in color, size, and shape with clean edges
Prescribed by licensed physician with documented thyroid lab work
Stored in original container at room temperature, protected from moisture and light

Warning Signs

Research chemical or compounded liquid T3 without third-party potency verification
Tablets that crumble easily or show discoloration (potential degradation)
Product sourced internationally without verification of manufacturer legitimacy

Bad Signs

No labeling, incorrect strength labeling, or missing expiration date
Tablets with inconsistent sizing or visible damage
Product sourced from unverified supplier with no testing documentation
Counterfeit packaging or misspelled brand names

References

Liothyronine Treatment of Hypothyroidism: A Systematic Review and Meta-Analysis

Jonklaas, J., Bianco, A.C., Bauer, A.J., et al.

The Journal of Clinical Endocrinology & Metabolism (2014)

Systematic review of T3 and T3/T4 combination therapy for hypothyroidism. Found that while T3 monotherapy normalizes thyroid markers, combination T4/T3 therapy may benefit a subset of patients with persistent symptoms on T4 alone, particularly those with DIO2 polymorphisms.
Guidelines for the Treatment of Hypothyroidism: American Thyroid Association Task Force

Jonklaas, J., Bianco, A.C., Bauer, A.J., et al.

Thyroid (2014)

Comprehensive ATA guidelines addressing the use of liothyronine in hypothyroidism management. Recommends levothyroxine as first-line therapy but acknowledges a role for T3 combination therapy in patients who remain symptomatic, with careful monitoring of TSH and free T3 levels.
Thyroid Hormone Action on Metabolism

Mullur, R., Liu, Y.Y., Brent, G.A.

Thyroid (2014)

Detailed review of thyroid hormone mechanisms including T3-mediated regulation of basal metabolic rate, thermogenesis via uncoupling proteins, lipid metabolism, and mitochondrial biogenesis. Clarifies the distinct roles of TR-alpha and TR-beta receptor isoforms in different tissues.
Type 2 Deiodinase Polymorphism and Response to Combined T4/T3 Therapy

Panicker, V., Saravanan, P., Vaidya, B., et al.

The Journal of Clinical Endocrinology & Metabolism (2009)

Demonstrated that patients carrying the DIO2 Thr92Ala polymorphism showed improved well-being and preference for combination T4/T3 therapy over T4 monotherapy, providing a genetic basis for individual variation in response to thyroid hormone replacement strategies.
Triiodothyronine (T3) and Body Composition: A Narrative Review of the Literature

De Pergola, G., Ciampolillo, A., Paolotti, S., et al.

International Journal of Molecular Sciences (2023)

Review of T3's role in body composition regulation, including its effects on resting energy expenditure, lipid oxidation, and the metabolic consequences of both hypothyroid and hyperthyroid states. Highlights the dose-dependent shift from anabolic to catabolic effects.

Disclaimer

This information is for educational and research purposes only. Consult a healthcare professional before use.