Levothyroxine Therapy

Hypothyroidism reflects inadequate thyroid hormone production. In primary hypothyroidism, reduced circulating free T4 removes negative feedback from the pituitary, so TSH rises as the body attempts to stimulate the failing thyroid gland. Levothyroxine is synthetic T4. It restores the circulating hormone supply and provides substrate for peripheral conversion to the more active hormone, T3. Replacement works gradually because thyroid hormone acts through intracellular receptors and because the pituitary response takes time to equilibrate. The usual laboratory pattern for overt primary hypothyroidism is TSH greater than 4.50 µIU/mL with a low free T4. Subclinical hypothyroidism has a TSH of 4.50–10.00 µIU/mL with a normal free T4. Oral levothyroxine tablets are first-line therapy for overt disease and for most patients with subclinical disease.