There have been some reports that metformin can decrease TSH levels in people with an underactive thyroid or Hashimoto's thyroiditis. These reports suggest that metformin may suppress TSH levels, perhaps without effecting thyroid hormone levels. However, more studies involving larger groups of people are needed before any conclusions can be made about metformin, TSH level, and thyroid hormone levels.
The article, published in the January 2006 edition of The Journal of Clinical Endocrinology and Metabolism, discussed four case reports. These four patients had been taking levothyroxine (Levothroid®, Levoxyl®, Synthroid®, Tirosint®, Unithroid®) and had been quite stable for years. When these patients started taking metformin, their TSH levels decreased to below-normal levels.
In one patient, the decreased TSH was not accompanied by increased thyroid hormone levels (free T4 levels), as would be expected. TSH and thyroid hormone levels usually change in opposite directions; TSH levels usually decrease when thyroid hormones increase, and vice versa. This effect is usually so predictable that testing TSH levels is usually done in place of testing the actual thyroid hormone levels.
This study suggests that metformin may suppress TSH levels, perhaps with no effect on thyroid hormone levels. This may have some usefulness as a treatment for certain medical conditions, or it may cause problems with TSH monitoring in people taking metformin. However, more studies involving larger groups of people are needed before any conclusions can be made.
Written by/reviewed by: Kristi Monson, PharmD;Arthur Schoenstadt, MD
Last reviewed by: KristiMonson, PharmD;
List of references (click here):
Fortamet [package insert]. Ft. Lauderdale, FL: Andrx Labs, Inc.;2005 May.
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