Liothyronine (T3) is synthetic triiodothyronine — the biologically active thyroid hormone. At physiological concentrations it regulates basal metabolic rate through nuclear thyroid-receptor signalling and mitochondrial uncoupling-protein expression. At supra-physiological doses it drives the same pathways harder, producing fat-loss acceleration at the cost of protein catabolism, HPT-axis suppression, and dose-dependent cardiac strain.
Mechanism — why T3 is not “thyroid replacement”
Exogenous T3 bypasses the hepatic deiodinase conversion step that normally regulates systemic T3 availability from circulating T4. The feedback loop reads elevated T3 and suppresses TSH, which suppresses thyroid-gland T4 production. Over a 6–8 week course free T4 falls below reference range, free T3 rises above reference range, and TSH is undetectable. This is the expected picture on protocol — the concern arises at cessation.
Once T3 administration stops, the thyroid gland requires 4–12 weeks to resume adequate T4 output. During that window the user experiences iatrogenic hypothyroidism: fatigue, weight regain, cold intolerance, depression, poor recovery. Abrupt cessation of a high-dose T3 protocol produces the most severe rebound; tapered discontinuation produces a milder transition.
Dose ranges in published research
- Physiological replacement: 12.5–25 mcg/day (roughly the endogenous T3 equivalent from deiodinase conversion at eu-thyroid baseline).
- Fat-loss protocol, conservative: 25–37.5 mcg/day.
- Fat-loss protocol, moderate: 50 mcg/day.
- Aggressive — requires substantial anabolic support to prevent net catabolism: 50–75 mcg/day.
- Ceiling in any defensible research framework: 75 mcg/day. Beyond this, protein catabolism outpaces fat oxidation, HPT-axis suppression becomes slow to reverse, and cardiac β-receptor upregulation produces the stacking-risk pattern with clenbuterol.
Context: endogenous T3 production from thyroid and peripheral deiodinase conversion is approximately 30 mcg/day in a eu-thyroid adult male. 50 mcg exogenous doubles circulating T3 on top of what the body still produces, even with TSH suppression partially reducing endogenous output.
Taper schedule — never cold start, never cold stop
| Day range | Dose | Purpose |
|---|---|---|
| 1–5 | 12.5 mcg | Gradual receptor engagement |
| 6–10 | 25 mcg | Working dose approaching |
| 11–15 | 37.5 mcg | Near-target |
| 16–42 (cruise, 3–4 weeks) | 50 mcg | Active fat-loss phase |
| 43–47 | 37.5 mcg | Taper begin |
| 48–52 | 25 mcg | Taper middle |
| 53–57 | 12.5 mcg | Taper end |
| 58+ | Off | Monitor for rebound symptoms |
Total protocol length: 8 weeks. Longer exposures (12+ weeks) extend HPT-axis recovery time disproportionately — the 8-week window is a defensible compromise between fat-loss benefit and endocrine safety margin.
The anabolic-support requirement — non-negotiable
T3 upregulates both lipolytic and proteolytic pathways. Fat oxidation rises; muscle protein breakdown rises in parallel. Without exogenous androgen support, the net composition change is smaller-in-both-compartments rather than leaner-at-same-mass. This is the pharmacological basis for the reliable user observation that “T3 without test makes you small.”
Minimum anabolic floor during a T3 protocol: 150–200 mg/week testosterone (cruise dose) for nitrogen-balance preservation. Higher anabolic support allows higher T3 doses without net protein loss; the two pathways scale together.
Representative cutting stacks:
- Test E 200 mg/week + T3 50 mcg/day + clen 2-on/2-off (conservative).
- Test P 75 mg EOD + Tren A 50 mg EOD + T3 37.5 mcg/day (advanced recomp — note the lower T3 because the trenbolone AR activation is strongly anticatabolic).
- Test cruise 150 mg/week + Anavar 40–60 mg/day + T3 25 mcg/day (surgical cut, minimal stimulant load).
Side-effect profile
- Resting heart rate +10–15 bpm. Expected at 25–50 mcg. Above 100 bpm is a dose-reduction signal.
- Insomnia. Morning dosing only. Evening T3 reliably disrupts sleep via sympathetic tone elevation.
- Hyperhidrosis and heat intolerance. Expected; reflects elevated basal metabolic rate.
- Anxiety, fine tremor, jitters. Indicates over-dose or individual thyroid-receptor hypersensitivity. Reduce 12.5 mcg, reassess 5 days.
- Palpitations, chest tightness, arrhythmia symptoms. Immediate dose reduction; consider cessation. T3 stacked with clenbuterol at high doses is the combination producing the worst cardiac outcomes in the exposure literature.
Bloodwork — what to order and what to expect
Pre-cycle: TSH, free T3, free T4. Identifies pre-existing thyroid dysfunction that would alter protocol risk. Sub-clinical hypothyroidism (TSH 4–10 mIU/L with free T4 low-normal) shifts the calculus — supplementation addresses the pre-existing pattern, but the dosing framework differs from protocol use in a eu-thyroid subject.
Week 4 of cycle: TSH suppressed to <0.1 mIU/L (expected). Free T4 low or below reference (expected — the gland is idle). Free T3 upper-reference or above (expected — that is the administered hormone). If free T3 reads >6.0 pg/mL on the sensitive assay, reduce dose by 12.5 mcg and recheck in 10 days.
6 weeks post-cycle: TSH, free T3, free T4. The recovery panel. TSH above 4 mIU/L with free T3 below reference at week 6 post-PCT indicates inadequate thyroid recovery — add selenium 200 mcg/day and zinc 25 mg/day as deiodinase cofactor support, re-test at week 10. Persistent hypothyroidism at 3 months post-cycle warrants clinical endocrinology review.