The Effect of Dietary Supplements for Pregnant Women on Levothyroxine Absorption

Hypothyroidism is a common disease that is successfully treated with the replacement of thyroid hormones in the form of levothyroxine. Levothyroxine is drug used in the first line of substitution therapy because it appropriately mimics physiological secretion of thyroxine (T4) and its peripheral conversion to triiodothyronine (T3). However, due to its narrow therapeutic index, and because it is administered at very low doses (μg), it is particularly sensitive to the absorption phase interactions, which are due to its narrow therapeutic index, mainly clinically significant. In pregnancy, one of the goals is to maintain the values of thyroid stimulating hormone (TSH) in the lower part of the reference interval (below 2,5 mIU/L) for a proper child development. Also, due to increased nutritional needs during pregnancy, nutritional supplements are commonly used by pregnant women on their own initiative, but also often recommended by healthcare personnel. Reviewing the literature, we did not find high quality evidence to suggest the existence or to refute the interaction between dietary supplements for pregnant women and the absorption of levothyroxine. The aim of this study is to investigate this potential interaction between dietary supplements used in pregnancy and levothyroxine absorption in order to test the safety of the use of these preparations in pregnant women who are on replacement therapy with levothyroxine.

Primary goal is to asses if taking of the food supplement for pregnant women with levothyroxine diminishes the absorption of levothyroxine, which is measured through values of TSH, FT4 and FT3.

Secondary and other objectives are to determine whether the use of dietary supplements intended for pregnant women, due to its mineral-vitamin composition, can affect body weight, waist circumference and lipogram values of patients diagnosed with primary hypothyroidism.

This is a prospective randomized double-blind placebo-controlled clinical trial.

The study would include subjects diagnosed with primary hypothyroidism over the age of 18 and on stable LT4 therapy for at least 6 months. Although the study is intended to examine dietary supplements for pregnant women, the said population will be excluded from the study for safety reasons. Given that the mechanism of the vast majority of LT4 interactions is based on the adsorption of levothyroxine on the interfering substance, we believe that potential differences in LT4 absorption between pregnant women and the study population should not affect the clinical interpretation of the results of this study.