Thyroxine Absorption Test

Abstract

Poor compliance with Levothyroxine therapy leading to apparently 'resistant' hypothyroidism, termed 'pseudomalabsorption', is a common clinical conundrum.. A thyroxine absorption test may aid clinicians in confirming suspected poor compliance with greater objectivity, but there is a lack of uniformity in practice and interpretation of this test. We herein report a case of suspected pseudomalabsorption where a thyroxine absorption test helped to confirm the diagnosis. A 29-year-old female was diagnosed with primary hypothyroidism in 2009 with TSH at diagnosis of 180 microIU/L. Euthyroidism was achieved by Levothyroxine administration, which was gradually increased to 175mcg/day. In the subsequent 2 years, however, she remained persistently hypothyroid despite doses of Levothyroxine well exceeding her estimated weight adjusted requirement. A thyroxine absorption test was performed, where once weekly directly observed administration of a weight-adjusted dosage of levothyroxine was performed over a consecutive 4 week period. TSH fell from a baseline of 18 microIU/L to 0.8 microIU/L in the fifth week. This finding confirmed pseudomalabsorption and excluded the need for a potentially exhaustive search for an organic underlying cause for the persistently elevated TSH. Primary hypothyroidism is the most common cause of hypothyroidism, and typically responds well to oral Levothyroxine replacement therapy. Factors interfering with Levothyroxine absorption such as co-ingestion of foodstuffs and/or medications, or underlying gastroenterological conditions such as celiac disease, can result in apparent resistance to Levothyroxine therapy - with difficulty achieving a normal TSH and a requirement for high doses of levothyroxine. Poor compliance with Levothyroxine, if denied by the patient, can present with a similar clinical picture hence the term 'pseudomalabsorption' has been applied in such cases. In the scenario where pseudomalabsorption is suspected, but is refuted by the patient, a breakdown in the therapeutic relationship between the patient and the treating physician can easily develop