Short answer: this is a dangerous combination and I'd strongly advise against it

Why people do it

The core problem: T3/T4 is not fat-selective

• Cardiac: tachycardia, atrial fibrillation, and increased myocardial oxygen demand. Textbook pharmacology explicitly flags cardiac arrhythmia and even cardiac failure/death with thyroid hormone overdose (Kaplan & Sadock's Synopsis of Psychiatry). Liothyronine (T3) is specifically called out in emergency medicine texts as riskier than T4 in anyone with underlying cardiac disease because it can precipitate arrhythmias or infarction (Tintinalli's Emergency Medicine).
• Muscle/bone: increased protein turnover and bone resorption - counterproductive if the actual goal is preserving lean mass, and a long-term osteoporosis risk.
• HPT axis suppression: exogenous thyroid hormone suppresses TSH and endogenous thyroid output, similar in principle to how exogenous testosterone suppresses the HPG axis. Stopping abruptly can produce a rebound hypothyroid state.
• Periodic paralysis: there are documented case reports of bodybuilders on steroids plus unlabeled thyroid-hormone-containing "fat burner" supplements presenting with thyrotoxic periodic paralysis (severe hypokalemic weakness).

Why adding anabolic steroids makes it worse, not safer

• A 2024 case report described an acute severe tachyarrhythmia with underlying cardiomyopathy in a patient misusing anabolic-androgenic steroids together with thyroxine.
• A 2022 case report describes anabolic steroid misuse alone as a reversible cause of cardiomyopathy - reversible only if caught and stopped in time.
• A 2020 case report/series documented delayed diagnosis of covert T3 use in a bodybuilder presenting with thyrotoxicosis.
• A recent (2025) systematic review specifically covers poisoning from illegally used thyroid hormones.

My opinion

1. They need baseline and ongoing TSH/free T4/free T3, ECG, lipid panel, and echo if there's any cardiac symptom, ideally under an actual endocrinologist or physician who knows what's being taken.
2. Any resting tachycardia, palpitations, tremor, heat intolerance, or unexplained weakness needs urgent evaluation, not reassurance.
3. Fat loss goals are far better served by a caloric deficit, resistance training, and (if steroids are already a fixed decision the person won't reverse) at minimum getting real medical monitoring rather than self-dosing thyroid hormone from unregulated "fat burner" products, several of which have been found to contain undisclosed T3.

Side effects of using T3/T4 (beyond what's needed to treat actual hypothyroidism)

• Rapid heartbeat, palpitations, and increased risk of atrial fibrillation (Goodman & Gilman's, Katzung's Pharmacology)
• Elevated blood pressure
• Worst case: arrhythmias precipitating cardiac arrest or infarction, especially with T3 - emergency medicine texts specifically flag liothyronine as riskier than T4 in anyone with underlying heart disease (Tintinalli's Emergency Medicine)
• Documented case reports of thyrotoxic cardiomyopathy and severe tachyarrhythmia in people misusing thyroid hormone, particularly when combined with anabolic steroids

• Fine tremor, nervousness, anxiety, insomnia
• Heat intolerance and excessive sweating
• Muscle weakness/wasting - thyroid hormone excess accelerates protein catabolism, which works against muscle-building goals
• Rare but serious: thyrotoxic periodic paralysis (sudden severe muscle weakness from hypokalemia), reported in bodybuilders using covert thyroid hormone
• Increased bone resorption -> osteoporosis risk with prolonged use

• Increased bowel frequency/diarrhea
• Increased appetite paired with weight loss
• Menstrual irregularities in women
• If pushed to extremes: thyroid storm - fever, very rapid heart rate, vomiting, delirium - a life-threatening emergency

• Exogenous T3/T4 suppresses your own TSH and endogenous thyroid output. Stop suddenly and you can rebound into a hypothyroid state (fatigue, cold intolerance, weight regain) until your own axis recovers.

Recent research on thyroid hormone misuse in athletes

• Stuart et al., 2025 (Drug Testing and Analysis, PMID 40605400) - "Self-Reported Use of Thyroid Hormones by Athletes at the Olympic Games." Direct survey data on how many Olympic-level athletes self-report using T3/T4.
• Skrzypiec-Spring et al., 2025 - systematic review "Poisoning with Thyroid Hormones Used Illegally" (PMID 41471297) and a companion 2025 review "Autoimmune Thyroid Diseases and Physical Activity and Sports" (PMID 41153638), which cites survey data from Wroclaw, Poland gyms: 35% of amateur athletes who illegally used testosterone also used thyroid hormones, while thyroid hormone use overall was around 12% of trainees - reinforcing that T3/T4 is most often used alongside steroids, exactly the combination in your earlier question.
• Handelsman, Gild & Clifton-Bligh, 2023 (Journal of the Endocrine Society, PMID 36896254) - "Thyroid Hormone Abuse Among Elite Athletes," and their earlier 2022 companion piece in JCEM on the regulatory challenge - thyroid hormone is not currently WADA-banned, which they argue is a loophole given the doping potential.
• Di Girolamo et al., 2024 (Sports Medicine - Open, PMID 38536564) - "Metabolic Consequences of Anabolic Steroids, Insulin, and Growth Hormone Abuse in Recreational Bodybuilders" - relevant to the athlete biological passport approach for detecting this kind of stacking.
• Ovadia et al., 2025 (Harefuah, PMID 40530630) - case report of prolonged cardiac thyrotoxicosis from over-the-counter liothyronine, with explicit policy recommendations for regulators.
• Medscape's 2024 National Poison Data System summary (cited in my search) notes male patients were significantly more likely than females to misuse thyroid hormone specifically for doping/physique purposes, and states plainly: "evidence supporting performance or metabolic benefits is minimal, whereas the adverse effects of excess thyroid hormone exposure are well documented."

Does T3/T4 alone (no steroids) still cause side effects?

• Tachycardia, palpitations, risk of atrial fibrillation, increased cardiac workload
• Tremor, anxiety, insomnia, heat intolerance, sweating
• Increased bowel frequency, appetite changes
• Muscle protein catabolism (works against the physique goal) and bone resorption with prolonged use
• TSH/HPT axis suppression, with rebound hypothyroid symptoms on stopping
• Rare but serious: thyrotoxic periodic paralysis (documented repeatedly in bodybuilders, e.g. PMID 32984532, 33869098, 34612209), thyroid storm in extreme overdose

On dosing measurement

1. Baseline labs before touching anything: TSH, free T4, free T3, resting heart rate, blood pressure, and an ECG if you have any cardiac history or symptoms. If your TSH and free T4/T3 are already normal, there is no physiological deficiency to correct - anything you take from there is pushing you into iatrogenic hyperthyroidism, not "optimizing" a hormone level.
2. TSH is the primary monitoring marker. A suppressed TSH (<0.1 mIU/L) is a direct biochemical marker of overtreatment/thyrotoxicosis, per Katzung's Pharmacology - this is the number clinicians watch when someone is at risk of overtreatment, not just how you feel.
3. Symptom monitoring: resting heart rate above your normal baseline, palpitations, hand tremor, unexplained sweating, or diarrhea are your body telling you free hormone levels are too high - these typically show up before a lab test would catch it.
4. Frequency: in legitimate clinical titration (e.g. hypothyroid patients being started on levothyroxine), labs are rechecked roughly every 4-6 weeks after any dose change, because T4 takes weeks to reach steady state; T3 has a much shorter half-life, so its levels swing sharply within a single day, making home "dosing by feel" especially unreliable and part of why T3 shows up disproportionately in the toxicity case reports.
5. There's no upper "training wheels" dose I can responsibly give you because individual thyroid sensitivity varies substantially, and the entire premise of using it without an actual deficiency means you're titrating toward harm, not toward a therapeutic target.