Synthetic T4 → converted peripherally to active T3 → restores the euthyroid metabolic state.
Black Box Warning
Not for the treatment of obesity or weight loss — ineffective in euthyroid patients, and large doses can cause serious or life-threatening toxicity, especially with sympathomimetic (anorectic) amines.
Clinical Notes
Hypothyroidism. Take on an empty stomach, separate from calcium/iron. Over-replacement → AFib/osteoporosis. Recheck TSH in 6–8 weeks.