Overview
Replacement Physiology and Appropriate Use
Hypothyroidism occurs when the thyroid does not produce enough thyroxine (T4) and triiodothyronine (T3).
Hypothyroidism occurs when the thyroid does not produce enough thyroxine (T4) and triiodothyronine (T3). Falling circulating thyroid hormone removes normal feedback on the pituitary, so thyroid-stimulating hormone (TSH) rises as the body tries to stimulate the thyroid gland. Levothyroxine is synthetic T4. After absorption, body tissues convert enough T4 to active T3 to restore metabolic activity and pituitary feedback. Replacement works slowly because thyroid hormone has a long physiologic effect; a patient should not expect fatigue, constipation, cold intolerance, or dry skin to resolve after one dose. Overt hypothyroidism is associated with TSH greater than 4.50 µIU/mL and a low free T4. Subclinical hypothyroidism has TSH from 4.50–10.00 µIU/mL with a normal free T4. Levothyroxine tablets are first-line replacement for overt hypothyroidism and for most patients with subclinical disease when treatment is prescribed.