Rapid Levothyroxine Absorption Testing: A Case Series of Nonadherent Patients

BACKGROUND Nonadherence to levothyroxine therapy is one cause of persistent hypothyroidism. To distinguish nonadherence from malabsorption, a levothyroxine absorption test is required. Typically, this test measures the serum free thyroxine (FT4) response to 1000 mcg of oral levothyroxine over 4 to 24 hours. Published data indicate that serum levels of FT4 are at or near their peak 2 hours after levothyroxine ingestion. OBJECTIVES We present the successful completion of 2-hour levothyroxine absorption testing in 3 patients as a retrospective case series. PATIENTS AND METHODS Serum levels of thyroid stimulating hormone (TSH), FT4, and free triiodothyronine (FT3) were drawn at 0, 60, and 120 minutes after 1000 mcg of oral levothyroxine. RESULTS In all 3 cases, baseline thyroid function indicated the patients had taken their prescribed doses of levothyroxine prior to the absorption test. Despite high baseline levels both FT3 and FT4 increased during each absorption test, providing more evidence of adequate levothyroxine absorption. Subsequently, patients achieved normal TSH levels on lower doses of levothyroxine. CONCLUSIONS Levothyroxine absorption testing over 2 hours may offer a more rapid alternative to the commonly used longer protocols to rule out malabsorption. Scheduling a levothyroxine absorption test may induce some patients to start adhering to levothyroxine therapy.