Topic: subclinical hypothyroidism


Title: Metformin may inhibit thyrotropin (TSH) secretion in
patients with hypothyroidism treated with thyroxine.

Authors: Vigersky RA, Filmore-Nassar A, Glass AR (Washington DC, USA)
Reference: Journal of Clinical Endocrinology and Metabolism: 91: 225-227, 2006

Context: 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 years 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 4
patients with chronic hypothyroidism, previously euthyroid on fixed doses of l-T4 for several years, in
whom the metformin was initiated.
Design: This was a retrospective review.

The study was conducted at a tertiary care military hospital (Walter Reed Army Medical
Center) providing care to active-duty soldiers, sailors and marines, retirees of the armed forces, and
their eligible dependents.
Four patients with chronic hypothyroidism who were placed on metformin participated
in the study.

Intervention, Main Outcome Measurement:
Serum TSH, free T4, and free T3 levels were measured
during metformin treatment.

Initiation of treatment with metformin (three for diabetes mellitus and one for non-alcoholic
steato-hepatitis) 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.

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 present 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.

The four patients described in this
daily and it was found (eight months later) that his serum TSH had decreased from 1.6 females (age range: 58-75 yrs), chronically to 0.11 mU/L. Patient 2 was given 500 mg treated with l-T4 after radioiodine ablation of metformin daily and it was found (two for Graves’ disease (1), for Hashimoto’s thyroiditis (1), and after thyroidectomy (2). decreased from 0.64 to 0.31 mU/L. Patient Patient 1 was given 1500 mg of metformin and it was found (interval not given) that properties, alterations in thyroid hormones found (three months later) that her serum TSH had decreased from 1.9 to 0.39 mU/L. inhibition of TSH secretion (dopamine-like effect). Despite the limitations, this study patients, with inherent weaknesses mainly has the interest of showing that drugs may because it was retrospective. Serum free T4 administration. Patients were studied at different time points while taking different doses of metformin. Overall, the changes in serum TSH were rather small. If true, (Daniel Glinoer, MD; PhD)
the effects of metformin to reduce serum
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