Levothyroxine is the standard thyroid hormone replacement treatment for primary hypothyroidism. In adults with overt primary hypothyroidism who require full thyroid hormone replacement, a commonly used weight based estimate is approximately 1.6 mcg/kg/day.

The 1.6 mcg/kg/day estimate is mainly relevant when full thyroid hormone replacement is required. Body weight alone does not determine the appropriate levothyroxine dose.

Age, lean body mass, degree of TSH elevation, residual thyroid function, cause of hypothyroidism, cardiovascular disease, pregnancy, medications, absorption, and the intended TSH target may influence the final dose.

The calculated full replacement dose should not automatically be used as the initial dose in older adults or patients with cardiovascular disease.

Patients with known coronary artery disease should generally be started with a low levothyroxine dose, commonly 12.5 to 25 mcg/day, followed by gradual dose increases according to symptoms and serum TSH response.

Actual body weight based dosing may be less accurate in patients with obesity. Observational studies have shown that levothyroxine requirements per kilogram of actual body weight tend to decrease as BMI increases, while lean body mass may have a stronger relationship with levothyroxine requirements.

This calculator should therefore be interpreted cautiously in patients with overweight or obesity.

Patients with subclinical hypothyroidism generally require lower doses than patients with overt hypothyroidism who require complete replacement.

Lower doses, such as 25 to 50 mcg/day, may be appropriate in milder hypothyroidism, including some patients with TSH levels of 10 mIU/L or lower. The 1.6 mcg/kg/day calculation should therefore not be automatically applied to subclinical hypothyroidism.

Pregnancy requires specific levothyroxine management and should not be managed using this calculator alone. Pregnancy may increase levothyroxine requirements, and thyroid function should be monitored closely according to pregnancy specific recommendations.

Serum TSH should generally be reassessed approximately 4 to 6 weeks after initiation of levothyroxine or after a dose change. Dose adjustments are commonly made in increments of approximately 12.5 to 25 mcg/day according to the TSH response, clinical status, age, and cardiovascular risk.

Levothyroxine absorption can be affected by food, calcium, iron, antacids, bile acid sequestrants, and other medications. Consistent administration and appropriate separation from interfering substances are important when interpreting the TSH response and determining the maintenance dose.