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Fat BurnerNot WADA ProhibitedCompare

T3 (Cytomel)

Also known as: Liothyronine, Cytomel, Triiodothyronine, T3

5
1 · Natty510 · Enhanced

Lower numbers = closer to natural. Higher numbers = more enhanced.

Why this rating?

Thyroid receptors bind T3 with much higher affinity than T4, making T3 roughly 3-4x more potent at the cellular level (FDA label; NCBI thyroid physiology reviews). A classic VLCD trial (Wilson & Lamberts, Int J Obes 1981) showed T3 raised daily weight loss from 269±27 g/day to 395±32 g/day, a real, significant metabolic enhancement. But T3 is a naturally occurring hormone, not an anabolic: at supraphysiological doses it measurably increases muscle protein breakdown (Burman et al., Metabolism 1979, found ~60% higher 3-methylhistidine excretion at 100mcg/day vs. fasting alone), and the FDA explicitly warns against its use for weight loss in euthyroid individuals. Notably not WADA-prohibited. Rated 5 as an aggressive metabolic/cutting enhancer rather than a muscle-building compound.

Overview

The most biologically active thyroid hormone, roughly 3-4x more potent than T4 at the cellular level. Used in bodybuilding to dramatically accelerate metabolic rate and fat loss during cutting phases. Powerful but carries significant risks including muscle catabolism and thyroid suppression if used improperly.

Important Warnings

  • •FDA explicitly warns AGAINST use for weight loss in normal thyroid individuals
  • •Muscle catabolism is SIGNIFICANT at doses above 75mcg/day
  • •MUST taper down - abrupt cessation causes rebound hypothyroidism
  • •Do not use if you have cardiovascular issues (increases heart failure/stroke risk)
  • •Limit cycles to 4-8 weeks maximum
  • •High protein intake absolutely essential
  • •Not recommended for those with existing bone density issues
  • •Monitor thyroid function (TSH, Free T3, Free T4) with bloodwork
  • •Weight regain common (10-30%) post-cycle if diet not adjusted

Purpose & Use Cases

Fat Loss Acceleration

Dramatically increases metabolic rate and caloric expenditure. A controlled VLCD trial (Wilson & Lamberts 1981) showed T3 increased daily weight loss from 269g/day to 395g/day.

Contest Prep

Used by competitive bodybuilders in the final weeks before competition to achieve extremely low body fat levels.

Metabolic Enhancement

Increases BMR by stimulating cellular oxidation, enhancing carbohydrate metabolism, and improving thermogenesis.

Counteracting Metabolic Adaptation

Can prevent the metabolic slowdown that occurs during prolonged caloric restriction.

Benefits

  • Roughly 3-4x more potent than T4 at cellular level
  • Rapid onset - effects begin within hours
  • Dramatically increases basal metabolic rate
  • Enhances fat oxidation and thermogenesis
  • Short half-life allows quick adjustment if issues arise
  • Can prevent metabolic adaptation during extended dieting
  • Improves lipid profiles (lowers LDL, total cholesterol)
  • Not a controlled substance - prescription only

Good to Know

It suppresses the thyroid axis: taper, never stop cold

Exogenous T3 shuts down TSH and your own T4-to-T3 conversion via negative feedback. Stopping abruptly leaves you transiently hypothyroid (crushing fatigue, sluggish metabolism and fast fat regain) until the axis restarts. That is why T3 must be tapered down over the last week or two, not dropped. Most users recover the axis in roughly 2-4 weeks; longer or higher-dose use risks slower recovery. This is a thyroid-axis issue, distinct from the testosterone/PCT question, no SERM helps here.

Above ~75mcg it starts eating muscle

T3 is catabolic at supraphysiological doses: a controlled fasting study found 100mcg/day raised 3-methylhistidine (a muscle-breakdown marker) roughly 60% above fasting-alone levels. This is why "more is better" backfires, past ~75mcg you increasingly burn lean tissue, not just fat. Keeping the dose moderate, protein high and running it alongside anabolics is how competitors limit the damage.

T3 vs T4: active hormone vs slow prohormone

T3 (liothyronine) is the active hormone, roughly 3-4x more potent than T4 with a fast onset, which is why it is favoured for cutting but also why it hits the heart and the axis harder. T4 (levothyroxine) is a slower, gentler prohormone the body converts as needed; some prefer low-dose T4 or a T4/T3 blend to be easier on the thyroid axis at the cost of speed.

One of the few physique drugs NOT banned by WADA: but the cardiac risk is real

As a thyroid hormone, T3 is not on the WADA Prohibited List, unlike most cutting drugs. That is not a safety endorsement: T3 raises heart rate, and hyperthyroid-level dosing is linked to meaningfully higher stroke and cardiovascular mortality risk in the endocrine literature, so it is a poor choice for anyone with cardiovascular issues.

Dosage Guidelines

Experience LevelDosage Range
Beginner25 – 50 mcg/day
Intermediate50 – 75 mcg/day
Advanced75 – 100 mcg/day
Frequency
Once daily or split into 2 doses. Take in the morning.
Typical Cycle Length
4 – 8 weeks
Notes

Start at 25mcg/day, increase by 12.5-25mcg every 5-7 days. The FDA hypothyroidism label tops maintenance dosing out at 75mcg/day; bodybuilding cutting protocols sometimes push to 100mcg/day, but muscle catabolism rises sharply above ~75mcg. MUST taper down at end of cycle - do not stop abruptly. Protein intake 1.6-2.2g/kg essential to minimize muscle loss.

Half-Life

Plasma half-life is ~22h (~1 day) in single-dose PK studies; the FDA label cites a ~2.5-day biological half-life. Clearance is faster in hyperthyroid states and slower in hypothyroid states.

Side Effects

Muscle Catabolism

common
Severity
4/5

At supraphysiological doses, T3 significantly increases muscle protein breakdown. A controlled fasting study found 100mcg/day of T3 for 5 days raised 3-methylhistidine excretion (a muscle-breakdown marker) roughly 60% above fasting-alone levels, rising further after treatment stopped (Burman et al., Metabolism 1979).

Mitigation

Keep doses moderate (≤75mcg). Stack with anabolic steroids to offset catabolism. High protein intake essential (1.6-2.2g/kg). Avoid extreme caloric deficits.

Heart Palpitations/Tachycardia

common
Severity
3.5/5

Very common due to direct T3 impact on cardiac myocytes and increased catecholamine sensitivity. A large meta-analysis found overt hyperthyroidism, the physiological state high-dose T3 use mimics, associated with roughly 35% higher stroke risk and 20% higher cardiovascular mortality vs. euthyroid controls (Sohn et al., Endocrinol Metab 2020).

Mitigation

Start low, titrate slowly. Monitor heart rate. Discontinue if severe arrhythmia develops.

Thyroid Suppression

common
Severity
3.5/5

Exogenous T3 suppresses natural TSH and thyroid hormone production via negative feedback. Risk increases beyond 4-6 weeks of use.

Mitigation

Limit cycles to 4-8 weeks. MUST taper down - do not stop abruptly. Allow 2-8 weeks for full thyroid recovery.

Anxiety/Nervousness

common
Severity
2.5/5

Increased catecholamine sensitivity causes anxiety, tremors, and shakiness.

Mitigation

Reduce dose if severe. Usually manageable at moderate doses.

Insomnia

common
Severity
2/5

Stimulant-like effects can disrupt sleep, especially if dosed late in day.

Mitigation

Take T3 in the morning. Avoid afternoon/evening dosing.

Bone Density Loss

uncommon
Severity
3.5/5

T3 directly stimulates bone resorption through TRα receptors. High T3 associated with low bone density, particularly at hip.

Mitigation

Limit cycle length. Not recommended for long-term use. Post-menopausal women at higher risk.

Heat Intolerance/Sweating

common
Severity
1.5/5

Increased thermogenesis causes excessive sweating and difficulty tolerating warm temperatures.

Mitigation

Stay hydrated. Adjust environment as needed. Sign that T3 is working.

General Mitigation Strategies

The key concern with T3 is muscle catabolism - this is dose-dependent and significantly worse above 75mcg/day. Always stack with anabolic compounds to preserve muscle. Taper down at end of cycle - do NOT stop abruptly. Allow thyroid to recover between cycles. Protein intake must be high. Monitor thyroid function with bloodwork.

Support Supplements

Ancillary supplements commonly run alongside T3 (Cytomel) to manage side effects, support the target tissue, or fill nutrient demands it creates.

High protein intake

Key

The primary defense against T3's catabolism. Supraphysiological T3 accelerates muscle protein breakdown, so a high protein intake is required to blunt net muscle loss. This matters more than any pill.

Dose
1.6-2.2 g/kg bodyweight per day
Timing
Spread across meals

Low-dose beta-blocker (e.g. propranolol / nebivolol)

Blunts the palpitations, tremor and tachycardia from T3's catecholamine-sensitising effect. Propranolol additionally reduces peripheral T4-to-T3 conversion.

Dose
Per prescriber; titrate to heart rate/symptoms
Timing
Daily while symptomatic
When
Only if palpitations/tachycardia are problematic and cardiac status has been cleared. Not needed for most low-dose users.

Selenium

Cofactor for the deiodinase enzymes that govern thyroid hormone conversion and for thyroid antioxidant defence. Modest, supportive role: not a substitute for tapering.

Dose
100-200 mcg/day
Timing
Daily with food
When
General thyroid support; weak evidence for changing outcomes on exogenous T3 specifically.

Post Cycle Therapy (PCT)

✓PCT Not Required

T3 does not affect the HPT (testosterone) axis. However, it does suppress the hypothalamic-pituitary-thyroid axis. No traditional PCT needed but MUST taper dose and allow thyroid recovery. Older TRH-stimulation studies of thyroid hormone withdrawal found the pituitary TSH response returning within roughly 1-2 weeks in many patients, with full normalization of the axis taking longer. Exact timelines are dose- and duration-dependent and not tightly established for bodybuilding-style protocols, so treat "2-8 weeks" as the working estimate.

How It Works

T3 diffuses into the cell nucleus and binds to thyroid receptor proteins attached to DNA. This hormone-nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins. Increases basal metabolic rate (BMR), enhances catecholamine sensitivity, accelerates cellular oxidation, and increases energy expenditure through thyroid hormone receptor alpha (TRα) in skeletal muscle. At supraphysiological doses, catabolism predominates over anabolism.

Fundamentals

Reference on the practices relevant to T3 (Cytomel): how they are done and where they go wrong. Not a recommendation to use it.

Bloodwork: what to measure and when→

The markers that matter here

Warning signs & when to seek care→

When to stop and get help

Common Stacks

  • T3 + Clenbuterol - Classic cutting stack, enhanced thermogenesis
  • T3 + Anavar - Offset muscle catabolism while cutting
  • T3 + Testosterone - Prevent muscle loss during aggressive cut
  • T3 + HGH - Synergistic fat burning with minimal catabolism

WADA Status

✓Not Prohibited by WADA

T3 (Liothyronine/Cytomel) is not currently listed on the WADA Prohibited List. It is a thyroid hormone used to treat hypothyroidism.

References

Last updated: July 18, 2026