Endocrine Abstracts

Introduction: Hypothyroidism is a very common condition, and its treatment is relatively easy with levothyroxine (LT4). However, cases of resistant hypothyroidism were reported despite high doses of levothyroxine. The aim of this study was to evaluate the usefulness of the levothyroxine absorption test to confirm or exclude a disorder of thyroid hormone absorption.

Methods: This was a retrospective study including patients who were admitted to our department between January 2018 and December 2021 for resistant hypothyroidism under high-dose of levothyroxine (≥ 2.7μg/kg/day). Levothyroxine absorption test was performed on all patients. The percentage of levothyroxine absorption was calculated using the following formula: % LT4 absorption=[(peak ΔT4 × volume distribution) / administered dose of LT4 (μg)] × 100 (Volume distribution (dL)=4.42 × body mass index). Normal absorption was defied by a % LT4 absorption > 60 %.

Results: Seven patients (5 women and 2 men) were enrolled in this study. Their mean age was of 39 ± 11.3 years [27-62]. Their mean body weight was 85.4 kg [72-98] with a mean body mass index of 30.3 kg/m² [25.4-36]. The average duration of hypothyroidism was 10 years [2-20]. The mean dose of levothyroxine was 5.3 μg/kg/day. At baseline (T0), the mean TSH level was 278.8 mIU/l (nr: 0.35- 4.95) and the mean FT4 level was 0.59 ng/dl (nr: 0.7-1.5). During the levothyroxine absorption test, the mean peak of FT4 was 0.80 ng/dl. The average % LT4 absorption was 4.5%. It was < 60% in all patients consistent with the diagnosis of malabsorption. Etiological investigations showed negative celiac disease serology for all patients and a helicobacter pylori gastritis in six patients.

Conclusion: Our results illustrate the interest of the levothyroxine absorption test to confirm the diagnosis of malabsorption and avoid diagnosing wrongly a pseudo-malabsorption.