Levothyroxine Dosage and Estrogens

"Please let me have more information than just that there is a relationship. Are there reasons for a rise in TSH levels other than being given estrogen while on levothyroxine?" -- JG, Foster City, CA

Estrogens increase thyroxine-binding globulin, a protein which binds over 99% of the thyroid hormone present in the blood. Thyroid hormone bound to TBG represents a circulating storage pool of thyroid hormone and is not readily available to other organs such as liver and kidney. The thyroid hormone that is unbound or "free" is readily available to other tissues.

In normal individuals, the increase in TBG results in a transient increased production of thyroid hormone to saturate the extra TBG in the blood and maintain the normal level of unbound thyroid hormone.

In patients who are being treated with thyroid tablets because their own thyroid is unable to produce a normal amount of thyroid hormone, it is possible that a higher dose of thyroid hormone might be necessary after beginning estrogen therapy. It is uncertain how commonly this occurs. Women taking thyroid hormone for hypothyroidism who become pregnant may require as much as 45% more thyroxine - however other factors besides higher estrogen levels may be involved.

Important Update: July, 2002
Karen Traite, TFA Staff

In July, 2002, the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (NIH) called an early halt to the Women's Health Initiative (WHI) study of the risks and benefits of combined estrogen and progestin in healthy menopausal women. The study was stopped due to an increased risk of invasive breast cancer. Study participants taking estrogen plus progestin were also found to have increases in coronary heart disease, stroke, and pulmonary embolism compared to women taking placebo pills.

You can read the press release from the National Institutes of Health about why the study was halted here.

The results of this study were published in the Journal of the American Medical Association (JAMA). Risks and Benefits of Estrogen Plus Progestin in Healthy Menopausal Women JAMA 2002; 288: 321-333. You can find this article on JAMA's website.

If you are taking estrogen plus progestin to combat the symptoms of menopause and are concerned by this update, please discuss those concerns with your doctor. It is very important that you not decide to stop taking any medication without first discussing it with your physician. There may be dangers to suddenly stopping a medication that you are unaware of.