Thyrotropin suppression by metformin.
BACKGROUND
Drug-drug interactions are common but often are discovered only long after initial drug release. Metformin has been available in the United States for 9 yr and elsewhere for many years, but as of yet there are no reports that the drug modifies thyroid hormone economy.


OBJECTIVE
The objective of the study was to describe the clinical and biochemical findings of four patients with chronic hypothyroidism, previously euthyroid on fixed doses of L-T4 for several years, in whom the metformin was initiated.


METHODS
This was a retrospective review.


METHODS
The study was conducted at a tertiary care military hospital providing care to active-duty soldiers, sailors, and marines, retirees of the armed forces, and their eligible dependents.


METHODS
Four patients with chronic hypothyroidism who were placed on metformin participated in the study. INTERVENTION, MAIN OUTCOME MEASURE: Serum TSH, free T4, and free T3 levels were measured during metformin treatment.


RESULTS
Initiation of treatment with metformin (three for diabetes mellitus and one for nonalcoholic steatohepatitis) caused suppression of TSH to subnormal levels without clinical symptoms of hyperthyroidism in any patients. There was no change in free T4 or free T3 in patient 1.


CONCLUSIONS
No other potential causes of TSH suppression, including medication changes or interference in the TSH assay, could be identified. The mechanism of the fall in serum TSH in these four patients is unclear at this time. Should these findings be confirmed in larger prospective studies, metformin's ability to suppress TSH without causing clinical or chemical hyperthyroidism might render this drug a useful adjunct to the treatment of patients with thyroid cancer.
Vigersky
Robert A
RA
Endocrinology Service, Walter Reed, Army Medical Center, Washington, DC 20307, USA. robert.vigersky@na.amedd.army.mil
Filmore-Nassar
Amy
A
Glass
Allan R
AR
D000368:Aged; D003924:Diabetes Mellitus, Type 2 / Q000150:complications / Q000188:drug therapy; D003928:Diabetic Nephropathies / Q000188:drug therapy; D004347:Drug Interactions; D005260:Female; D006042:Goiter / Q000601:surgery; D006111:Graves Disease / Q000150:complications / Q000532:radiotherapy; D050031:Hashimoto Disease / Q000097:blood; D006801:Humans; D007004:Hypoglycemic Agents / Q000009:adverse effects* / Q000627:therapeutic use; D007037:Hypothyroidism / Q000188:drug therapy; D008111:Liver Function Tests; D008297:Male; D008687:Metformin / Q000009:adverse effects* / Q000627:therapeutic use; D008875:Middle Aged; D012189:Retrospective Studies; D013960:Thyroid Function Tests; D013965:Thyroidectomy; D013972:Thyrotropin / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D014284:Triiodothyronine / Q000097:blood
D007004:Hypoglycemic Agents; D014284:Triiodothyronine; D013972:Thyrotropin; D008687:Metformin