Understanding Metformin and Your Thyroid

A Promising Potential Addition to Thyroid Treatment

metformin, glucophage, thyroid, hypothyroidism, thyroid cancer
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One promising new finding in the study of thyroid disease is an understanding of the relationship between the popular type 2 diabetes drug metformin (brand name Glucophage) and the thyroid.

Metformin is used to treat type 2 diabetes and insulin resistance. There is a well-understood link between insulin resistance—higher-than-normal levels of insulin, with correspondingly higher blood glucose levels that indicate the insulin is not effective at reducing blood sugar—and not only a larger thyroid gland (goiter), but a higher prevalence of both thyroid nodules and thyroid cancer.

Interestingly, new research published in the Journal of Endocrinology has found that patients treated with metformin have a smaller thyroid size and lower TSH level, but also reduced risk of thyroid cancer, thyroid nodules, and goiters.

About Metformin

Metformin, which is sold under the brand name Glucophage, is considered the first-line drug used to treat type 2 diabetes, especially in patients who are overweight. Metformin is a biguanide, meaning that it decreases your liver's production of glucose, and increases your body’s sensitivity to insulin. The United States started using metformin in 1995, and metformin is now the most widely-used oral drug for type 2 diabetes worldwide. In the United States, the typical cost of metformin therapy is from $5 to $25 per month.

Metformin and Your Thyroid

The large study reported on in the Journal of Endocrinology looked at the relationship between metformin and thyroid function and summarized a number of important effects of metformin that have implications for thyroid treatment in the future.

Specifically, the study reported on several key findings:

  • Metformin and Thyroid Cancer: Numerous research findings show that metformin slows the growth of thyroid cancers, and has what is known as anti-proliferative activity. Metformin significantly reduced the ability of insulin to stimulate the growth of a variety of thyroid cells, and thyroid cancer cells, including the notoriously resistant anaplastic thyroid cancer cells. Metformin also inhibits growth and prevents metastases in medullary thyroid cancer. The metformin appears to interfere with key signaling pathways that promote such growth.
  • Metformin and Thyroid Nodules: Metformin also has anti-proliferative activity against thyroid nodules, slowing their growth or reducing their size. The study pointed to research that found that in patients with small benign thyroid nodules and insulin resistance, treatment with metformin resulted in a reduction in the size of the nodules, as well as a drop in TSH levels. Another recent study of women with insulin resistance and thyroid nodules found that there was a significantly greater reduction in nodule size in the patients treated with metformin and levothyroxine, versus either drug taken alone.
  • Metformin and TSH Suppression for Thyroid Cancer Patients: Metformin treatment was consistently associated with decreased levels of TSH, suggesting that it may have a role in current therapies for thyroid cancer survivors that are targeted at suppressing TSH levels to prevent thyroid cancer recurrence. The metformin may be particularly useful, because it appears to suppress TSH levels, without causing hyperthyroidism.
  • Metformin and Thyroid Conditions in Type 2 Diabetics: There is statistically lower prevalence of diagnosed hypothyroidism in type 2 diabetics on metformin therapy. Metformin is associated with reduced levels of thyroid stimulating hormone (TSH) in diabetic patients. In type 2 diabetic patients also coping with thyroid cancer, treatment with metformin was associated with higher remission and survival rates in diabetic patients with thyroid cancer, and more favorable outcomes in diabetic patients with differentiated thyroid cancer that had metastasized to the lymph nodes. The researchers theorize that metformin enhances the effects of thyroid hormones on the pituitary gland.

    The Key Considerations

    Studies have shown that levothyroxine absorption is not changed by taking metformin, so it is safe to take the two drugs concurrently.

    A challenge, however, is that because metformin can lower TSH without affecting the T4 or T3 levels, it may not reflect the true thyroid status of type 2 diabetics with hypothyroidism who are taking both metformin and levothyroxine. It may also impair suppression of type 2 diabetics who are thyroid cancer survivors and who require not only suppressive TSH levels, but higher circulating levels of the T4 and T3 thyroid hormones to prevent cancer recurrence.

    According to the researchers:

    "Depressed TSH level provides false reassurance or may prompt initiation of therapy, or endocrinologists may decrease [levothyroxine] dosage with spuriously low TSH level. The clinicians should consider the effect of metformin when they interpret thyroid function to avoid any appropriate treatment or adjustment of [levothyroxine] dosage."

    Reducing the dosage may cause hypothyroidism symptoms to return because, despite a low TSH, there may not be sufficient circulating thyroid hormone to relieve hypothyroid symptoms.

    Additionally, after surgery to remove the gland to treat thyroid cancer, many patients are put on suppressive therapy, keeping the TSH level very low and thyroid hormone levels high to prevent thyroid cancer recurrence. For type 2 diabetics who have thyroid cancer, and have a thyroidectomy to remove the gland, an artificially low TSH due to metformin therapy may suggest a need for a reduced dose of levothyroxine. This reduction in dose, however, may not achieve suppression at the cellular level.

    A Word from Verywell

    These interesting new findings suggest that there may be a number of potential uses for metformin in several groups:

    • Type 2 diabetics with thyroid disease
    • Type 2 diabetics who do not have thyroid disease
    • People with hypothyroidism who have insulin resistance, metabolic syndrome, and/or obesity
    • Thyroid cancer survivors on TSH-suppressive therapy

    At this point, however, experts recommend more clinical studies to better evaluate the use of metformin as part of thyroid disease treatment and to establish potential treatment protocols and guidelines.

    If you are hypothyroid, and have insulin resistance, but are not yet taking metformin, it may be worthwhile to have a discussion with your physician regarding adding it to your treatment program. Metformin is considered to be a safe drug with few side effects, and is prescribed for metabolic syndrome/pre-diabetes/insulin resistance, and may halt progression to type 2 diabetes. These new findings suggest that it may also have a protective effect on your thyroid.

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    Article Sources
    • Distiller LA, et al. “Type 2 diabetes mellitus and hypothyroidism: the possible influence of metformin therapy.” Diabet Med 31 172-175. 2012.
    • Xianghui M et al. "Review: Metformin and thyroid disease." Journal of Endocrinology. Online prepublication, 14 February 2017. Abstract / Full PDF