levothyroxine

Levothyroxine is the lab-made copy of the body's main thyroid hormone, swallowed as a daily tablet to top up what an underactive thyroid cannot supply. For someone with hypothyroidism, it is essentially a replacement part: take the right amount and metabolism, energy, mood, and temperature regulation return to normal.

Chemically it is synthetic thyroxine (T4), identical to the hormone the gland would secrete. After absorption it serves as a steady reservoir that the body converts to the active form T3 in the tissues as needed. This conversion, plus its very long half-life of about a week, means blood levels stay smooth on once-daily dosing and a missed day matters little. It works through nuclear thyroid hormone receptors to restore normal gene expression and metabolic rate.

Dosing is individualized and checked by measuring thyroid-stimulating hormone in the blood, adjusting until it sits in the target range, with weeks needed between changes because of the long half-life. Absorption is finicky: it is best taken on an empty stomach, separated from food, calcium, iron, and certain other drugs that bind it in the gut. The main hazard is overtreatment, which mimics hyperthyroidism and over time can affect the heart and bones.