levothyroxine

Levothyroxine is a synthetic form of the thyroid hormone thyroxine (T4) used as replacement therapy for hypothyroidism in adults and children. It is also employed to suppress TSH in certain thyroid conditions after thyroidectomy or radioactive iodine treatment, and as part of therapy for differentiated thyroid cancer to reduce the risk of recurrence.

In the body, levothyroxine is converted to the more active triiodothyronine (T3). It acts by binding to thyroid hormone receptors in the cell nucleus to regulate metabolism and development. It has a long elimination half-life, about seven days in healthy adults, allowing once-daily dosing. The dose is individualized based on serum TSH and free T4, with gradual adjustments to reach the target range.

Administration is typically once daily on an empty stomach, 30 to 60 minutes before breakfast, with water. If morning dosing is not practical, bedtime dosing at least 4 hours after the last meal can be used. Pregnancy often increases the required dose due to changes in thyroid-binding proteins and hormone needs.

Monitoring and safety: therapy is usually lifelong for most patients. Efficacy is assessed by periodic measurement of TSH and free T4. Side effects reflect overreplacement (palpitations, anxiety, heat intolerance, weight loss). Long-term overtreatment increases the risk of atrial fibrillation and osteoporosis. Absorption can be reduced by calcium, iron, magnesium-containing antacids, sucralfate, and certain dietary minerals; dosing should be separated by several hours.

Special considerations: newborns with congenital hypothyroidism require early treatment. During pregnancy, levothyroxine is continued and doses are often increased. Brand and generic products are generally considered interchangeable, though small differences can occur. It is a prescription medication and should be used under medical supervision.