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 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. DESIGN: This was a retrospective review. SETTING: 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. PARTICIPANTS: 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.
Thank you so much for posting this; very interesting!
What this translates to is that it is SO important to get the free thyroid's measured (free T4 and free T3), not just a TSH, when getting evaluated for thyroid problems and when getting tested to monitor treatment, especially for those taking Metformin. If a doc only tests the TSH, it will look low/lower and may even seem to be in "hyper" territory when it really isn't. The free's will tell the real story. I wonder how many cysters on Met have had their thyroid meds reduced because their TSH looked artificially low.
Linda
__________________ dx pcos 1984, type II diabetes 2001, also hypertension
Met 2000mg since 2001, started Glucophage XR 4/22/04, then switched to Met ER 6/04; also: multi, Vit. C, Vit. E, B12/folic acid combo, fish oil & borage oil combo, garlic capsules, cinnamon, Vitex, calcium with magnesium/zinc, biotin, CoQ10, selenium,iron
Other meds: Verapamil and Altace(for blood pressure)
Started laser hair removal 7/29/03, completed 3/04 (it works!)
UAE for fibroid 3/24/03 and 3/16/04
Linda, Thank you for being such a great source of information yourself. I did a search because the Endo refused to treat antibodies and symptoms due to normal levels of TSH, T3, T4 and Frees. However, I had documentation that showed my TSH went from 3.0 to 0.64 in 6-weeks! I was shocked, matter of fact, all my levels dropped including the Frees but my antibodies were very high (Perioxidase 130and Thyroglobulin 94). I do take metformin (1500mg)for pcos but also take crestor (5mg)for cholesterol and several vitamins. My research also found that the crestor can lower TSH but usually only at the higher doses. I've got to wait 3 weeks to see PC but want to start a trial of thyroid hormone to treat the cause rather than individually treat the symptoms. Thanks again, Lisa
I am sorry, but I am really ignorant on the subject of what your levels are "normal" for the thyroid tests, so does this study indicate that Metformin can make your lab results look "better" than they are supposed to be, and mask the fact that you might have hypothyroidism. The reason I am asking is that my values tend to be borderline normal to normal, but I am on Met, and yet feel tired all the time, and I have low body temperature, and I am wondering if I might have a thyroid issue that might not be showing up on labs. Thanks!!!
I am sorry, but I am really ignorant on the subject of what your levels are "normal" for the thyroid tests, so does this study indicate that Metformin can make your lab results look "better" than they are supposed to be, and mask the fact that you might have hypothyroidism. The reason I am asking is that my values tend to be borderline normal to normal, but I am on Met, and yet feel tired all the time, and I have low body temperature, and I am wondering if I might have a thyroid issue that might not be showing up on labs. Thanks!!!
Yes, it's saying that Met can make your TSH look normal when it's not. However, a doc should never go by just a TSH anyway, although so many do. It's important to get your actual thyroid hormones tested (TSH is a pituitary hormone that regulates the thyroid, but it's not a thyroid hormone). You should request a free T4 and a free T3 to find out what's really going on. And your numbers should be at least midway in the lab range. Always request the actual lab report so you can see for yourself (and save for your records). Many docs will tell you your thyroid tests are normal by using just the TSH or maybe a Total T4,because so many of them aren't knowledgable enough about thyroid issues. A good doc will take everything into account, including your symptoms, risk for thyroid disorders, and the test results.
Linda
__________________ dx pcos 1984, type II diabetes 2001, also hypertension
Met 2000mg since 2001, started Glucophage XR 4/22/04, then switched to Met ER 6/04; also: multi, Vit. C, Vit. E, B12/folic acid combo, fish oil & borage oil combo, garlic capsules, cinnamon, Vitex, calcium with magnesium/zinc, biotin, CoQ10, selenium,iron
Other meds: Verapamil and Altace(for blood pressure)
Started laser hair removal 7/29/03, completed 3/04 (it works!)
UAE for fibroid 3/24/03 and 3/16/04
TSH suppression with Metformin 
