SAT-505 Non-Adherence to Levothyroxine Treatment, a Condition Not to Be Ignored nor Forgotten, Should Be Assessed by Thyroxine Absorption Test

Abstract Hypothyroidism due to non-compliance with levothyroxine (LT4) treatment is not infrequent (pseudomalabsorption). It should be considered in patients with persistent severe clinical and biochemical hypothyroidism even after excessive LT4 dose. The diagnosis can be confirmed by LT4 Absorption Test. We present 4 female patients (age range 21-44 years) with suspicion of (pseudo)malabsorption who underwent absorption test (3 patients with autoimmune hypothyroidism and one patient with hypothyroidism after total thyroidectomy due to Graves’ disease). They presented with persistent hypothyroidism (TSH 30 to >100 mU/L) even after gradual increase to excessive LT4 dose (400-700 μg daily). All denied non-compliance; drug and dietary interference with LT4 absorption and nephrotic syndrome were excluded. Two patients with autoimmune hypothyroidism underwent absorption test with their last daily LT4 dose as loading dose (700 and 200 μg) followed by hourly free T4 (fT4) determination for 6 hours, after an overnight fast. In one fT4 remained stable during the test (maximum fT4 increase +10% from baseline levels) indicating true malabsorption. New absorption test with combination of LT4 and ascorbic acid resulted in a fT4 raise +139% and further investigation revealed achlorhydria due to pernicious anaemia. The patient was treated with LT4 400μg x 2, Liothyronine 40μg x 2 and vitamin C in high doses. In the other patient, fT4 rose to maximum +71% from baseline 6 hours after the loading dose intake. She was diagnosed with pseudomalabsorption and became compliant and biochemically euthyroid with 150 μg/day LT4. Another two patients (1 with autoimmune, 1 with hypothyroidism after total thyroidectomy) underwent absorption test with a weight-adjusted weekly fasting LT4 dose (1,6 μg/kg of the body weight X 7) followed by hourly fT4 measurement for 5 hours. Peak fT4 reached a level of +290% and +309% of the baseline fT4 levels, respectively, 3 hours after administration of the dose. Both patients had pseudomalabsorption. They continued to deny non-compliance and were treated with once weekly supervised weight-adjusted LT4 over 6 consecutive weeks, resulting in TSH normalization. Pseudomalabsorption should be ruled out with LT4 absorption test in patients suspected of non-compliance with LT4 treatment, after drug/dietary interference, nephrotic syndrome and intestinal malabsorption are excluded. Different absorption protocols have been suggested with different loading doses (standard or weight-adjusted) and different duration (rapid 2-6 hours, long 5 weeks). An LT4 absorption peak with >70% increase in fT4 levels in 3 hours with a linear increase of fT4 in the first 1-1.5 hour is expected in the rapid test. In the long test normalization of TSH and fT4 is anticipated week 6 (1 week after the final dose). In case the patient remains non-compliant, treatment options include a single supervised weekly LT4 dose.