How T3 (Liothyronine) Affects Resting Heart Rate

T3 is the active thyroid hormone. In the heart it acts on the sinus node to raise the firing rate, increases how forcefully the muscle contracts, and increases the number and sensitivity of beta receptors, so adrenaline and stimulants have a bigger effect.

When T3 is taken on top of normal thyroid output for fat loss, TSH is suppressed and the excess acts like mild hyperthyroidism. Resting heart rate, heart rate at a given workload and the tendency to palpitations all rise.

Resting and 24-hour heart rate: on long-term TSH-suppressive thyroid hormone, 24-hour average heart rate was 84 bpm vs 70 bpm in controls (Biondi 1993, PMID 8345037). In subclinical hyperthyroidism, resting heart rate was 90.9 vs 78.9 bpm (Portella 2006, PMID 16910873).

Single dose: a single high dose of liothyronine did not produce an acute heart rate change in healthy volunteers (Chen 2022, PMID 35295986). The effect builds with sustained use.

Arrhythmia: subclinical hyperthyroidism roughly doubles the risk of atrial fibrillation (relative risk 1.99) (Singh 2024, PMID 38165911).

Physique use: no controlled data at bodybuilding doses (25-100 mcg/day). Forum reports describe palpitations and anxiety at 25-50 mcg.

Before starting: baseline TSH, free T3, free T4, and a 7-day average of morning resting heart rate.

While running it: log resting heart rate daily. Expect TSH to be suppressed and free T3 high; the heart rate trend tells you more about whether the dose is too much. Check potassium if you also run clenbuterol.

Cardio: zones based on heart rate will read high. Use the talk test and a fixed speed or wattage.

After stopping: thyroid output takes weeks to recover. Recheck TSH and free T4 4-6 weeks after the last dose. See low T3 on steroids for how AAS change thyroid results.

• Resting heart rate up 10 or more bpm over baseline, tremor, insomnia or palpitations: the dose is too high. Reduce it.
• Resting heart rate persistently above 100, an irregular pulse, chest pain or breathlessness: stop and see a doctor; ask for an ECG.
• Avoid stacking with clenbuterol, high-dose caffeine and stimulant pre-workouts, which amplify the thyroid effect on the heart.
• Do not mask it with a beta blocker. Beta blockers are used for genuine hyperthyroid symptom relief under a doctor, not to keep a fat-loss drug going.

Training zones on T3: why your zone 2 heart rate is wrong on clen, T3 and tren.