Rapid supervised levothyroxine absorption test in refractory hypothyroidism: suggestion for assessing absorption using two blood samples in low-resource settings

Refractory hypothyroidism is associated with high morbidity and increased healthcare expenditure. In general, the use of the levothyroxine absorption test looks promising in evaluating refractory hypothyroidism but has shown significant variability in protocols in multiple settings. We intended to assess the usefulness of the levothyroxine absorption test in a low-resource setting and to assess the factors associated with refractory hypothyroidism. A cross-sectional study among age-matched 25 cases of refractory hypothyroidism and 24 treatment-responsive hypothyroid controls was conducted. A supervised levothyroxine absorption test was performed with levothyroxine 1000 μg tablets after a 10-h fast, and serum free tetraiodothyronine (FT4) levels were measured at 0, 1, 2, 3, 4, and 5 h. Descriptive statistics, chi-square test, Student's t-test, and logistic regression were used in the analysis. Results showed no significant difference in age, body weight, etiology of hypothyroidism, interfering medications, thyroxine storage, and ingestion technique in cases and controls. Cases had a longer duration of hypothyroidism and males had a higher peak FT4 concentration. During pooled analysis, serum FT4 peaked at 3 h with an increment of 149.4% (128.4-170.5%) from baseline and plateaued thereafter. The absolute value of FT4 at 3 h was 41.59 (s.d. 14.14) pmol/L (3.23 ng/dL). We concluded that there was no significant difference in the pattern of levothyroxine absorption in both groups. The most common cause of refractory disease was pseudo-malabsorption. Rapid supervised levothyroxine absorption test with two blood samples for FT4 at baseline and at the peak of absorption (3 h) is simple, convenient, and cost-effective, particularly in low-resource settings.