The Effects of Simvastatin on the Serum Concentrations of Thyroid Stimulating Hormone and Free Thyroxine in Hypothyroid Patients Treated with Levothyroxine

Background: Statins, such as simvastatin, are the drugs of choice for the treatment of hypercholesterolemia. On the other hand hypercholesterolmia can occur in hypothyroid patients, who receive levothyroxine. There are few clinical case reports in regards to drug interaction between levothyroxine and lovastatin or simvastatin, indicating decreased levothyroxine effects. This study aimed at determining possible interaction between simvastatin and levothyroxine in hypothyroid patients by assessing serum levels of thyroid stimulating hormone (TSH) and free thyroxine (FT4), the two important laboratory indices for levothyroxine therapy. Methods: In a cross sectional study, 41 eligible hypothyroid patients receiving levothyroxine (50-150 µg/d) were selected. Blood samples were taken before and after three months of simultaneous treatment with simvastatin (20 mg/d) and levothyroxine to determine the serum levels of TSH and FT4. Results: There was no significant difference between the serum levels of TSH (P=0.77) or FT4 (P=0.76) before and after three months of simultaneous treatment. Also, there was no aggravation or initiation of any sign or symptom of hypothyroidism in the patients during the study period. Conclusion: Considering that FT(4) and TSH are the most reliable indicators for the levothyroxine treatment, the findings of the present study suggest that there may not be any significant interaction between simvastatin and levothyroxine.