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Metformin’s Effect on Levothyroxine Absorption

Aug 14, 2015

Scientists still struggling to find exact mechanism responsible for altered TSH levels when taking drug…

A recent study was published in Thyroid evaluating 26 healthy adults ages 18 to 50 years old who had no use of levothyroxine or metformin in the prior 3 months. They were looking to see if metformin had any effect on intestinal absorption of levothyroxine.


Levothyroxine absorption is affected by a variety of minerals, medications and nutrients. Previous studies have shown metformin alters TSH levels in patients. Researchers in this study wanted to see if metformin’s alteration of TSH levels was due to enhancing levothyroxine’s absorption.

Participants in this study fasted and were given 600 micrograms of levothyroxine orally. Serum concentrations of thyroxine were measured at baseline and at several intervals through six hours after administration. Measures were taken before and after a week of 850mg metformin was given three times daily (the maximum dose of metformin).

On average, these participants received 96.5% of the metformin doses. Researchers found no significant difference in total T4 before (12.03 ± 2.03 ug/dL) and after metformin therapy (11.63 ± 1.92 ug/dL).

Sadly, this study suggests metformin’s effects on TSH levels is not due to changes in levothyroxine absorption. Now further research is necessary to discover the mechanism behind altered TSH levels.

Perhaps the answer lies within the hypothalamic-pituitary-axis where researchers suggest some other possible theories point.

Practice Pearls:

  • Researchers have been baffled by the mechanism through which metformin alters TSH levels.
  • By studying concurrent T4 levels in healthy patients taking both metformin and levothyroxine, researchers found no significant impact of metformin on levothyroxine’s absorption.
  • Additional theories, such as that involving the hypothalamic-pituitary-axis, will need to be further explored to solve the mystery of why metformin alters TSH levels.

Al-Alusi, Mostafa, Lin Du, Ning Li, et al. “Metformin Does Not Suppress Serum Thyrotropin by Increasing Levothyroxine Absorption.” Thyroid (2015): 150720092749005. Web. 5 Aug. 2015.