Appropriate replacement dose of thyroxine in primary hypothyroidism

Tablet formulation of levothyroxine is absorbed less well than powdered levothyroxine

The comparative bioavailability of oral doses of levothyroxine (LT(4)) formulation taken as tablets, after being crushed, or chewed before swallowing has not been well studied. Three patients with hypothyroidism who showed persistent elevation of serum thyrotropin (TSH) despite taking 200, 150, and 125 microg of LT(4) tablets per day are presented. They did not show signs and symptoms of gastrointestinal illness that could interfere with the absorption of LT(4) nor history of such a condition. They did not concurrently take medications known to affect the absorption of LT(4) from the gut. Their serum TSH levels normalized when the tablets were taken after being pulverized. The difference appeared attributable to the slow dissolution of the tablets in the gut of these patients.