Lewotyroksyn

Lewotyroksyn, known in English as levothyroxine, is a synthetic form of thyroxine (T4) used as a replacement therapy for hypothyroidism and for suppression of thyroid tissue in certain conditions. It is identical to the natural hormone and is converted in the body to the metabolically active triiodothyronine (T3), helping regulate metabolism, growth, and development.

Indications include primary hypothyroidism (due to autoimmune disease, thyroid surgery, or radioiodine treatment), congenital hypothyroidism, and, in some cases, goiter or benign thyroid nodules where tissue suppression is desired.

Administration is typically once daily, in the morning on an empty stomach with water, at least 30 minutes before food. Dosing is individualized and adjusted based on TSH and free T4 measurements, generally every 4-6 weeks when changing dose.

Starting doses commonly range from 25-50 micrograms daily for adults, with smaller doses for older patients or those with heart disease. The goal is normalization of TSH within the reference range. In pregnancy, dose adjustments are common to accommodate changes in thyroid hormone requirements.

Adverse effects resemble thyroid excess and include palpitations, anxiety, tremor, heat intolerance, and insomnia. Long-term overtreatment may increase the risk of atrial fibrillation and bone loss in susceptible individuals.

Important interactions include reduced absorption with calcium, iron, aluminum-containing antacids, sucralfate, and cholestyramine. Drugs such as rifampin, carbamazepine, and some antiseizure medications can increase clearance. Estrogen-containing therapies raise thyroxine-binding globulin levels, affecting dosing and monitoring.

Monitoring relies on serum TSH and free T4 levels, typically 6-8 weeks after a dose change, then every 6-12 months once stable.