Endocrine Abstracts

JOINT1818

Introduction: Levothyroxine is the mainstay treatment of hypothyroidism. However, more than 15% of levothyroxine-treated patients failed to achieve the recommended serum TSH, due to either malabsorption, increased metabolism of thyroxine (T4) or non-adherence to treatment. A levothyroxine absorption test can differentiate true malabsorption from non-adherence or pseudo-malabsorption. Multiple protocols of levothyroxine absorption test have been published with differences in test doses, formulations, test duration, frequency of blood collection, absolute or relative peak or increments of total or free T4, and thresholds for normal absorption.

Patients and Methods: 143 patients (109 women), mean (±SE) age 42±1 years, body mass index 25.5±0.5 kg/m², were treated after total thyroidectomy for benign (n = 71) or malignant (n = 41) diseases, autoimmune thyroiditis (n = 29), congenital (n = 5), central (n = 4) and post-radioiodine (n = 3) hypothyroidism, with levothyroxine alone (n = 137) or in association with liothyronine (n = 6). The mean daily dose of levothyroxine was 3.27±0.9 μg/kg. After an overnight fast, the first blood sample (baseline) was taken; then patients absorbed the daily levothyroxine dose and blood samples were drawn every 2 hours during 24 hours. Serum TT4 and FT4 concentrations were evaluated in all samples, TSH concentrations were measured before and 24-hour after levothyroxine absorption.

Results: Prior intake of levothyroxine, baseline TT4, FT4 and TSH concentrations were 7.6 ± 0.2 μg/dl, 12.5 ± 0.4 pg/ml and 25.1 ± 3.3 mU/l respectively, demonstrating refractory primary hypothyroidism (n = 139). After orally levothyroxine intake, TT4 and FT4 concentrations increased at 9.4 ± 0.2 μg/dl and 15.8 ± 0.5 pg/ml (P < 10^-3), respectively. TT4 and FT4 levels peaked after 4.2 ± 0.2 and 4.3 ± 0.3 hours, respectively. TT4 increments were diminished in chronic gastritis (n = 19, 1.20 ± 0.23 μg/dl, P < 0.01), Helicobacter pylori infection (n = 15, 1.58 ± 0.2 μg/dl, P = 0.02) and proton pump inhibitor treatment (n = 9, 1.74 ± 0.2 μg/dl, ns) attesting decreased levothyroxine absorption in such patients. There were no adverse events reported by the patients during the levothyroxine absorption test.

Conclusion: Levothyroxine absorption test can be achieved via the absorption of daily dosage of levothyroxine with the evaluation of TT4/FT4 levels over 4 – 6 hours follow-up. The test is clinically well tolerated. In clinical practice, more studies are necessary in patients with refractory hypothyroidism due to either factors interfering with levothyroxine absorption or T4 metabolism or to pseudo-malabsorption.