The thyroid gland produces hormones (T4 and T3) that regulate metabolism, energy production, body temperature, heart rate, and virtually every organ system. When thyroid function goes awry — too little (hypothyroidism) or too much (hyperthyroidism) — medication becomes essential.

Levothyroxine (synthetic T4) is the standard of care for hypothyroidism and is one of the top 3 most prescribed medications in many countries. It replaces the T4 that the thyroid gland can no longer produce adequately. The body converts T4 to the more active T3 in peripheral tissues. Dosing is individualized based on body weight (typically 1.6 mcg/kg/day for complete replacement) and adjusted based on TSH levels every 6–8 weeks until stable.

Levothyroxine absorption is significantly affected by food and other medications. Best practice: take it on an empty stomach, 30–60 minutes before breakfast, with water only. Calcium supplements, iron supplements, antacids, and proton pump inhibitors all reduce absorption and should be separated by at least 4 hours. Coffee and high-fiber diets also impair absorption. Even switching between different levothyroxine brands can alter bioavailability and require dose adjustment.

Some patients remain symptomatic despite normal TSH levels on levothyroxine. Liothyronine (synthetic T3) can be added in some cases, though guidelines generally do not recommend routine combination therapy due to T3's short half-life and the risk of cardiac effects. Desiccated thyroid extract (from porcine thyroid) contains both T4 and T3 and has a dedicated following, though standardization concerns exist.

Methimazole (or carbimazole in some countries) is the first-line antithyroid medication. It inhibits thyroid peroxidase, blocking the synthesis of new thyroid hormones. Propylthiouracil (PTU) is an alternative, preferred in the first trimester of pregnancy and in thyroid storm. The most serious risk of antithyroid drugs is agranulocytosis (severe drop in white blood cells), occurring in about 0.1–0.3% of patients. Warning signs include sore throat, fever, and mouth ulcers.

Hypothyroidism typically requires lifelong levothyroxine replacement. TSH should be checked annually once the dose is stable, and more frequently during pregnancy (when requirements increase by 30–50%), after medication changes, or if symptoms recur. For hyperthyroidism, antithyroid drugs are usually given for 12–18 months, with radioactive iodine or surgery as definitive alternatives.

Seek medical attention if you experience rapid heart rate, unexplained weight loss with increased appetite, heat intolerance, or eye changes (possible hyperthyroidism), or if you notice increasing fatigue, weight gain, cold intolerance, constipation, or depression despite being on levothyroxine. Pregnant women should have thyroid function tested early and monitored throughout pregnancy.