Diagnosis of Levothyroxine Pseudo-malabsorption: The Results of Standardised Testing Protocol

The levothyroxine absorption test for the assessment of pseudo-malabsorption in patients with refractory hypothyroidism has not been standardised. The aim of this observational study was to describe a protocol for levothyroxine (LT-4) absorption test in patients with refractory hypothyroidism, to report possible side effects and to emphasise the importance of pseudo-malabsorption in the differential diagnosis. The results of 10 patients, who underwent LT-4 absorption tests because thyroid stimulating hormone suppression could not be achieved despite the need for LT-4 >3 mcg/kg/day, were retrospectively analysed. When compared with basal free T4 (fT4), a statistically significant increase in fT4 was observed after the first hour (p=0.012). fT4 reached its peak level and plateau at the 4th hour. The fT4 peak level increased 3.25 times compared to baseline. The shorter LT-4 absorption test with low doses may provide an alternative method to the commonly used longer protocols with higher doses to rule out malabsorption. Key Words: Levothyroxine absorption test, Pseudo-malabsorption, Resistant hypothyroidism.