Levothyroxine Pseudomalabsorption and Thyroxine Absorption Testing with Use of High-dose Levothyroxine: Case Report and Discussion

Objective: To report the case of a 55-year-old woman who had been prescribed a daily dose of 1,000 μg of levothyroxine for the treatment of hypothyroidism but still had severe biochemical hypothyroidism and to discuss the use of thyroxine absorption testing to diagnose pseudomalabsorption.

Methods: The patient was admitted to the hospital for supervised thyroxine absorption testing. Baseline thyroid function tests were performed. An oral dose of 1,000 μg of levothyroxine was administered while the patient had an empty stomach, and thyroid function tests were repeated at 2, 4, and 6 hours after administration. She was also given all her prescribed antihypertensive medications, and the blood pressure (which had been persistently high) was measured every 2 hours.

Results: After administration of 1,000 μg of levothyroxine, a rapid improvement in the results of her thyroid function tests was noted. Similarly, a rapid decrease in her blood pressure was observed after supervised administration of her antihypertensive medications. A diagnosis of nonadherence to treatment (pseudomalabsorption of levothyroxine) was made. After reduction of her levothyroxine dosage to 100 μg daily, results of thyroid function tests showed improvement. The doses of her antihypertensive medications were likewise altered.

Conclusion: We suggest that patients who are receiving doses of levothyroxine of more than 2 μg/kg of body weight, with persistently increased thyroid-stimulating hormone levels, should undergo testing for malabsorption and pseudomalabsorption of levothyroxine. Thyroxine absorption testing with use of high-dose levothyroxine is useful in diagnosing pseudomalabsorption but needs formal evaluation and validation.