Validity of rapid free T4 testing to distinguish levothyroxine non compliance from malabsorption: a case report

Abstract

Refractory hypothyroidism is characterised by persistently elevated thyroid-stimulating hormone (TSH) despite adequate or high-dose levothyroxine therapy. It presents a common clinical challenge, often requiring evaluation for causes such as poor adherence, drug interactions, or gastrointestinal malabsorption. Pseudomalabsorption due to inconsistent medication intake is the most frequent and reversible cause. Differentiating this from true malabsorption is crucial, as the latter may necessitate investigation for conditions like celiac disease, inflammatory bowel disease, or post-surgical states. The levothyroxine absorption test (LT4AT) is commonly used for this purpose, but conventional protocols are lengthy, costly, and inconvenient. Recently, simplified approaches using a single high oral dose with early measurement of serum free thyroxine (FT4) have been proposed. A rise in FT4 within a few hours indicates adequate intestinal absorption. We report a 54-year-old woman with refractory hypothyroidism in whom a short-duration LT4AT confirmed adequate absorption, suggesting pseudomalabsorption.