Evaluation of sodium levothyroxine absorption in real clinical practice using a single-bolus oral test

BACKGROUND: Thyroid hormone replacement therapy results remain unsatisfactory in 30-50% of cases. To differentiate between true levothyroxine sodium malabsorption and pseudomalabsorption due to poor compliance, several absorption assessment tests with different evaluation criteria are proposed. AIM: To determine the criteria for normal absorption of levothyroxine sodium when performing an oral test with a single bolus dose of 600 mсg. MATERIALS AND METHODS: The study involved 20 healthy volunteers of both sexes, aged 18 to 35 years, with normal body weight. The bolus dose of levothyroxine sodium was 600 mсg. Blood samples for determination of free thyroxine (fT4) concentration in serum were taken on an empty stomach, 1 hour, 2, 3, 4, 6 hours after taking the drug. RESULTS: After taking levothyroxine sodium at a dose of 600 mcg, the maximum level of free T4 was observed after 2 hours - 21,00 pmol/l [19,20; 23,16]. The minimum increase in the concentration of free T4 2 hours after taking levothyroxine sodium at a dose of 600 mcg was - 18,0%, the maximum - 91,1%. CONCLUSION: To assess the absorption of levothyroxine sodium, a single oral dose of 600 mcg of levothyroxine sodium is recommended. Absorption of levothyroxine sodium can be considered normal if the increase in free T4 concentration 2, 3, or 4 hours after the bolus dose is at least 18%.