Adults

Initial daily dose typically 12.5-50 mcg, adjusted by 12.5-25 mcg increments every 4-6 weeks based on TSH levels. Maintenance dose usually 50-200 mcg daily.

Elderly

Lower initial doses (e.g., 12.5-25 mcg daily) are recommended, with careful titration due to increased sensitivity to thyroid hormones and potential cardiac risk.

Renal_impairment

No specific dose adjustment is generally required for patients with renal impairment, as levothyroxine is mainly metabolized in the liver.

Onset

The therapeutic effect is gradual, with full clinical response often observed over several weeks.

Excretion

Excreted primarily through the kidneys (approximately 80%) and bile (approximately 20%).

Half life

The average plasma half-life of levothyroxine is approximately 6-7 days, allowing for once-daily dosing.

Absorption

Oral absorption is incomplete and variable (40-80%), influenced by food, gastric pH, and co-administered drugs. Peak plasma concentrations occur 2-4 hours after administration.

Metabolism

Levothyroxine is primarily metabolized in the liver and kidney by deiodination (conversion to T3) and conjugation (glucuronidation and sulfation).

Warfarin

Levothyroxine can potentiate the effects of oral anticoagulants like warfarin, requiring closer INR monitoring and possible dose reduction of warfarin.

Beta-blockers

May reduce the therapeutic effects of beta-blockers when thyroid status is normalized.

Certain antidepressants (e.g., tricyclic antidepressants)

May increase the risk of cardiac arrhythmias.

Antacids (e.g., aluminum and magnesium hydroxide) & Iron/Calcium supplements

Decrease levothyroxine absorption. Separate administration by at least 4 hours.

Bile acid sequestrants (e.g., cholestyramine, colestipol) & Ion exchange resins

Decrease levothyroxine absorption. Administer levothyroxine 4-5 hours before or after these drugs.