Levothyroxine absorption test in patients with refractory hypothyroidism: how to interpret patient's response to the test?

INTRODUCTION

Levothyroxine (LT4) absorption test is indicated in patients with refractory hypothyroidism to distinguish between pseudo-malabsorption and malabsorption. However, this test is not standardized and its interpretation remains controversial. The aim of the present study was to evaluate the different interpretations of the LT4 absorption test to confirm or infirm a disorder of LT4 absorption in patients with refractory hypothyroidism.

METHODS

This was a retrospective study including patients who were admitted for refractory hypothyroidism. LT4 absorption test was performed in all patients. Four different equations were used to interpret the test: The LT4 absorption rate, FT4 increment, FT4 increase rate, and FT4 ratio. Celiac disease serology and digestive endoscopy with biopsy were performed in all patients.

RESULTS

Seven patients were enrolled in this study. Their median age was 38 years. The median duration of hypothyroidism was 2.6 years. The median dose of LT4 was 4 µg/kg/day. Before the LT4 absorption test, the median TSH and FT4 levels were 72.59 mIU/L and 0.52 ng/dL, respectively. Using LT4 absorption rate and FT4 ratio formulas, the test was consistent with the diagnosis of malabsorption in all patients. However, when using FT4 increase rate, four patients (P1, P2, P4, and P5) had malabsorption and three patients (P3, P6, and P7) had pseudo-malabsorption. According to the FT4 increment, six patients (P1, P2, P4, P5, P6, and P7) had malabsorption and one patient (P3) had pseudo-malabsorption. Etiological investigations revealed the presence of Helicobacter pylori gastritis in four patients (P1, P2, P4, and P5). Patients 3, 6, and 7 admitted non-compliance with LT4 treatment when confronted with evidence about pseudo-malabsorption.

CONCLUSION

The FT4 increase rate seemed more effective than LT4 absorption rate, FT4 ratio, and FT4 increment in differentiating pseudo-malabsorption from malabsorption. However, further multicenter studies involving larger sample sizes would be useful to confirm these findings.