Thyroid abnormalities are common endocrine disorders in the United States, for which thyroid hormones are frequently prescribed. Levothyroxine is often considered the thyroid hormone of choice due to its long half-life, stability, uniform potency, lack of allergenic foreign protein, and relative low cost. Thyroid function test (FT4, TT4, TT3 and TSH) are used to monitor efficacy of levothyroxine therapy. Studies have suggested that levothyroxine has a 9-hour distribution phase, resulting in a 9% to 55% increase in FT4 and TT4 and a 23% to 40% decrease in TSH following levothyroxine administration. Although these studies have suggested monitoring therapy based on trough TFTs (blood sampling prior levothyroxine administration), the state-of-the-art practice is to assess TFTs post levothyroxine administration. Further, it is unclear whether the changes in TSH are due to drug administration or related to normal diurnal variation. This randomized, prospective, open-label, cross-over study, involving a total of 28 patients receiving levothyroxine therapy, will further evaluate the effects of levothyroxine administration time on TFTs. Patients will be randomly assigned to either Group A (7 replacement and 7 suppression patients, who will not take their usual levothyroxine does prior to serial TFTs) or Group B (7 replacement and 7 suppression patients, who will not take their usual levothyroxine dose until after serial blood sampling for determination of TFTs). After a wash-out period of not less than one week, patients will be crossed over to the alternate group.
