Metformin therapy did not change levothyroxine absorption in healthy adults, according to recent study findings published in Thyroid.
Decreased serum thyroid-stimulating hormone concentrations in patients who take levothyroxine and metformin, as observed from other studies, may be affected by mechanisms other than increased levothyroxine absorption, according to the researchers.
Mostafa Al-Alusi, of the David Geffen School of Medicine at UCLA, and colleagues evaluated 26 healthy euthyroid adults (54% men) aged 18 to 50 years without a history of recent levothyroxine or metformin use to determine the effect of metformin on intestinal levothyroxine absorption.
After an overnight fast, participants received a single dose of 600 µg levothyroxine orally. Measurements of serum total thyroxine concentrations were taken at baseline, 0.5, 1, 1.5, 2, 4 and 6 hours after administration. These measurements were taken before and after a week of metformin therapy (850 mg, three times a day).
Participants received 96.5% of the metformin doses.
No significant differences were found between mean serum T4 before (12.03 ± 2.03 µg/dL) and after metformin therapy (11.63 ± 1.92 µg/dL; P = .13).
After metformin, the area under the curve for serum total T4 was 3,765.45 ± 588.04 µg/dL-min compared with 3,893.19 ± 567.58 µg/dL-min before metformin (P = .09).
“Clinicians should recognize that some patients taking both metformin and levothyroxine may have lower serum TSH concentrations than expected, but the overall prevalence of and mechanism for this phenomenon require further study,” Al-Alusi told Endocrine Today. – by Amber Cox
Disclosure: The researchers report no relevant financial disclosures.