When researchers decided to explore the link between hormonal birth control and the risk of hypothyroidism, they made a surprising discovery: Contrary to earlier evidence, hormonal contraception decreases the risk of developing hypothyroidism.1
The researchers identified 18,578 women ages 18 to 45 years and divided them into these groups:
- Women using estrogen-progestin contraceptives;
- Women using progestin-only contraceptives;
- Women using progestin-containing intrauterine devices (IUD);
- Control.
The control group developed hypothyroidism at a rate that was significantly higher than the rate of those using hormonal contraception.
“We found that having a prior exposure to hormonal contraception, whether estrogen and progestin or just progestin IUDs, decreased risk of hypothyroidism later on,” says Ty Carroll, MD, study co-author and an associate professor in the division of endocrinology and molecular medicine at the Medical College of Wisconsin. “In our study, after our adjusted analysis, women who used estrogen and progestin contraception were only 30% as likely to develop hypothyroidism as controls. Women who use progestin-only oral or injectable contraception were only 14% as likely to develop hypothyroidism.”
People using progestin IUDs were 12% as likely to develop hypothyroidism. All the figures were statistically significant.
“The size of our study is impressive — far and away the largest in this type of topic that I’ve seen,” Carroll says. “Any kind of retrospective study’s results are not as good as a prospective study, and our results need to be confirmed with a prospective study.”
The findings are contrary to what prior studies have shown. For example, a 2021 study of 5,116 female adults who took birth control pills found a higher prevalence of hypothyroidism among those who had ever taken birth control pills.2
“One potential criticism is it’s hard to think of a mechanism why progestin IUDs would decrease the likelihood of developing hypothyroidism,” Carroll says. “It’s what the data show, but I can’t explain why that would be the case.”
Selection bias and method of contraception are possibilities. “One thing that stands out in our data is the IUD group is predominantly white, whereas the progestin-only oral contraceptive and injectable group is significantly more Black or African American than the group as a whole,” Carroll adds.
The 2021 study used national survey data and diagnosed hypothyroidism differently. Carroll and his colleague relied on the International Classification of Diseases 9th and/or 10th Revision codes in patients’ charts. The 2021 study used data on levothyroxine use or any thyroid-stimulating hormone level above normal.1,2
A prospective study might provide answers that practitioners could use in counseling patients about contraception use. “It’s a study that would be important and of interest to a lot of practitioners in the contraceptive world and thyroid world and general medical field,” Carroll says. “It would be important to know if hormonal contraception does impact the development of hypothyroidism.”
The new research should alleviate concerns clinicians might have about hypothyroidism risk among hormonal contraception users. Even if it is too early to say these contraceptives are beneficial at preventing hypothyroidism, there should be less concern about the potential of their increasing the risk of the disorder, Carroll notes.
Hypothyroidism is a common endocrine disorder. It affects women at five times the rate of men. An estimated 4.3% to 8.5% of the U.S. population is affected by subclinical hypothyroidism.1 Its symptoms are common, including fatigue, weight gain, cold intolerance, and abnormally dry skin. The symptoms are not specific, and laboratory testing is easy, so that is the way thyroid disorders are diagnosed, Carroll notes.
“Because the symptoms are so common and nonspecific, large-scale screening would not be recommended,” Carroll says.
REFERENCES
- Gloe S, Carroll TB. Hormonal contraceptive use is associated with a decreased incidence of hypothyroidism. Endocr Pract 2024;30:36-40.
- Qiu Y, Hu Y, Xing Z, et al. Birth control pills and risk of hypothyroidism: A cross-sectional study of the National Health and Nutrition Examination Survey, 2007-2012. BMJ Open 2021;11(6):e046607.
When researchers decided to explore the link between hormonal birth control and the risk of hypothyroidism, they made a surprising discovery: Contrary to earlier evidence, hormonal contraception decreases the risk of developing hypothyroidism.
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