Metformin may artificially lower your TSH

Ashweekins · 12-07-2007, 02:09 PM

http://www.thyroid.org/patients/note.../06_03_17.html

Metformin may inhibit thyrotropin secretion in patients with hypothyroidism treated with thyroxine

The background of the study.
Some drugs decrease the absorption or increase the metabolism of orally administered thyroxine (T4), thus lessening its actions. This report describes four patients taking T4 in whom co-administration of the antidiabetes drug metformin appeared to increase the action of T4, as manifested by a decrease in serum thyrotropin (TSH) concentrations.

Case Reports.
Patient 1 was a man with hypothyroidism who had been treated with 150 µg of T4 daily for four years. He was found to have nonalcoholic liver disease, and was treated with metformin, 1500 mg daily. Eight months later, his serum TSH was low (0.11 mU/L). Metformin was stopped. Two months later his serum TSH was normal (1.4 mU/L). Metformin was resumed, and his serum TSH was low again (0.11 mU/L). The drug was stopped again, after which his serum TSH increased to 0.93 mU/L.

Patient 2 was a woman with diabetes treated with insulin and hypothyroidism treated with 224 µg of T4 daily. Her serum TSH was 0.64 mU/L before and 0.31 mU/L after she had taken metformin, 500 mg daily, for two months, and it was 0.17 mU/L after the metformin dose was raised to 1000 mg daily.

Patient 3 was a man with diabetes treated with glipizide and hypothyroidism treated with 125 µg of T4 daily. His serum TSH was 1.3 mU/L before and 0.36 mU/L after he had taken metformin, 1000 mg daily. The T4 dose was reduced to 125 µg six days each week. After taking metformin for a total of six months, his serum TSH was 0.14 mU/L.

Patient 4 was a woman with diabetes treated with insulin and hypothyroidism treated with 175 µg of T4 daily. Her serum TSH was 1.9 mU/L before and 0.39 mU/L after treatment with metformin, 500 mg daily, for three months, and it was 1.2 mU/L four months after metformin was discontinued.

No patient had any clinical manifestations of thyroid dysfunction or changes in serum free T4 values while taking metformin. Addition of metformin to serum in vitro did not alter the measured serum TSH value.

The conclusions of the study.
Metformin slightly reduced serum TSH concentrations in patients with hypothyroidism who were taking T4.

The original article.
Vigersky RA, Filmore-Nassar A, Glass AR. Thyrotropin suppression by metformin. J Clin Endocrinol Metab 2006;91:225-7.