Endocrine Abstracts

We describe five patients (four female, one male) aged 24-58 years who, despite being prescribed oral levothyroxine (250-400 µg daily), remained biochemically hypothyroid and were referred for investigation by Endocrinology. Initial review included assessment for history of diarrhoea; coeliac screen; review of possible competing medications; and investigation of potential assay interference. After these causes of true malabsorption, physiological and analytical interference were excluded, patients underwent levothyroxine absorption test. Patients received a weight-dependent levothyroxine bolus (650-1900 µg) administered orally under supervision. Blood samples for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) were collected at baseline and hourly for at least four hours, FT4 concentration increase was assessed against published criteria. All patients demonstrated adequate levothyroxine absorption, exceeding criteria defined by Walker et al. (2013) and Caron et al. (2023); FT4 increase of >54% at 2 h, or FT4 +5.14 pmol/L at 4 h, respectively. Malabsorption was therefore excluded as the cause of refractory hypothyroidism, with poor compliance the most likely explanation. Weekly supervised bolus levothyroxine administration with baseline and 2 h bloods further demonstrated adequate absorption; significant improvement in TSH concentration was observed in four of five patients (mean pre-test TSH 187.3 mU/L vs mean TSH following final supervised dose 4.4 mU/L). Dose adjustments were made as required, one patient remained on a consolidated weekly levothyroxine dose due to persistent vomiting. At subsequent follow-up, four of five patients were biochemically euthyroid. Based on our experience, we have proposed a harmonised approach to levothyroxine absorption test at our centre: supervised weight-dependent levothyroxine dose with blood samples at baseline, and hourly up to 4 h. Adequate absorption is assessed using the criteria defined above, with malabsorption excluded if either criterion is met. Supervised levothyroxine administration with bloods at baseline and 2 h is continued for 5 weeks to ensure compliance.