Adults

Initial dose typically 25-50 mcg daily, adjusted by 12.5-25 mcg increments every 4-6 weeks based on TSH levels. Maintenance dose usually 75-125 mcg daily. For full replacement, about 1.6 mcg/kg/day.

Elderly

Lower initial dose (12.5-25 mcg daily) and slower titration due to increased sensitivity and higher risk of cardiac events. Adjust based on TSH levels.

Renal_impairment

No specific dose adjustment required for renal impairment. Monitor TSH levels as usual.

Onset

Therapeutic effects are gradual, usually taking several weeks to manifest fully, with a clinical response starting within 3-5 days.

Excretion

Mainly excreted in the urine (conjugated metabolites) and feces (unabsorbed drug and some metabolites). A small amount is excreted in bile.

Half life

Approximately 6-7 days (euthyroid patients), 9-10 days (hypothyroid patients).

Absorption

Variable, typically 40-80% absorbed from the GI tract, primarily in the jejunum and upper ileum. Absorption is affected by food, certain medications, and gastric pH.

Metabolism

Primarily deiodinated in the liver, kidney, and other peripheral tissues to T3 (active) and reverse T3 (inactive). Also undergoes some conjugation and enterohepatic recirculation.

Insulin, Oral Hypoglycemics

Increased dose requirement of antidiabetic agents may be needed.

Soy products, Dietary fiber

May decrease levothyroxine absorption. Monitor TSH levels.

Oral anticoagulants (e.g., Warfarin)

Levothyroxine may increase the anticoagulant effect. Monitor INR closely.

Cholestyramine, Colestipol, Sevelamer

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

Beta-blockers, Amiodarone, Propylthiouracil

May alter thyroid hormone metabolism or binding.

Antacids (aluminum and magnesium hydroxide), Calcium Carbonate, Iron Supplements

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