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 ½–1 h before breakfast, separate from calcium/iron. Over-replacement → AFib/↓ bone density. Recheck TSH in 4–6 weeks.