levothyroxine

Levothyroxine is a laboratory-made copy of thyroxine (T4), the main hormone the thyroid gland normally secretes. When the thyroid underperforms, a once-daily tablet of levothyroxine refills the body's thyroid hormone supply, restoring the metabolic 'thermostat' that governs heart rate, body temperature, digestion, mood, and energy.

Cleverly, the drug is the relatively inactive storage form of the hormone. The body's own tissues convert T4 into the more active T3 as needed, using deiodinase enzymes, which gives levothyroxine a long half-life and a smooth, steady effect — one reason a single daily dose can keep blood levels stable. It is best absorbed on an empty stomach, and certain foods, calcium, iron, and acid-reducing drugs can blunt its uptake.

Dosing is individualized and monitored by blood tests, chiefly TSH: too low a dose leaves the patient hypothyroid with a high TSH, while too high a dose causes a low TSH and symptoms of thyroid excess such as palpitations and bone loss. Because the right amount can change with weight, age, and especially pregnancy, levothyroxine therapy is periodically re-checked and re-titrated rather than set once and forgotten.