Thyroid Hormones (T3 & T4)

Thyroxine (T4) and the more active triiodothyronine (T3) are the body's primary metabolic throttle, influencing basal metabolic rate, body temperature, and heart rate. Hypothyroidism can lower BMR by roughly 5–15% and cause fatigue, cold intolerance, and mild weight gain — but once TSH is medicated back into range (typically 0.5–2.5 mIU/L), fat loss follows normal calorie-deficit rules, just sometimes at a marginally slower pace. Severe caloric restriction can itself suppress T3, one mechanism behind metabolic adaptation.