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What's the connection between the thyroid and fertility?

5 min read

The thyroid is a small gland located in your neck. Its job, as a crucial part of the endocrine system, is to control your body’s metabolism — the process by which your body converts what you eat and drink into energy — through the hormones it releases. The thyroid gets a message from the pituitary gland via thyroid-stimulating hormone (TSH) and releases triiodothyronine (T3), thyroxine (T4), and calcitonin.

While many are aware that an imbalance of TSH, T3, and T4 can cause weight or mood changes, did you know that it can also impact your menstrual cycle and fertility? In other words, your thyroid is something that’s worth paying attention to.

Keep reading to find out the answers to the following questions:

  • What are thyroid disorders?
  • How is thyroid dysfunction related to fertility?
  • Can thyroid dysfunction impact male fertility?
  • What can happen if you get pregnant with a thyroid disorder?
  • How can you minimize the risk of thyroid-related fertility issues?

First… a refresher on thyroid disorders

Thyroid disease is much more common in women than men — about five to eight times more common (the medical community is still not really sure why this is the case).

Hyperthyroidism (high levels of thyroid hormones due to an overactive thyroid gland) affects up to 5% of women. Symptoms of this condition can include unexplained weight loss, increased appetite, feelings of nervousness and anxiety, difficulty sleeping, fewer or lighter menstrual cycles, increased sweating, and heat intolerance.

Hypothyroidism (low levels of thyroid hormones due to an underactive thyroid gland) is found in 2-4% of women. Some common symptoms seen in hypothyroidism are weight gain, fatigue, constipation, feeling cold, thinning hair, pale skin, and increased or heavier menstrual bleeding. (Because of the similarity in symptoms between hypothyroidism and polycystic ovarian syndrome, or PCOS, providers will often check thyroid hormones when evaluating patients for PCOS, and vice versa.)

Proactive screening of thyroid hormone levels through a blood test is not currently routinely performed by doctors on nonpregnant women of reproductive age who aren’t experiencing symptoms. (Though the American Thyroid Association recommends looking into thyroid function regularly starting at age 35.) But if you have a family history of thyroid dysfunction, a personal or familial history of autoimmune disease (which can be related to thyroid function), or are suffering from symptoms of hypothyroidism or hyperthyroidism, it’s important to get your thyroid checked out.

If you’re wondering about your thyroid hormones — because you’re dealing with related symptoms or simply out of curiosity — we’ve got your back. With the Modern Fertility test, you can measure your TSH levels, among other reproductive hormones.

Thyroid dysfunction and reproductive health

Thyroid function is regulated by the hypothalamus-pituitary axis (HPA), an interconnected duo made up of the hypothalamus (a part of the brain that produces hormones) and the pituitary gland (which waits for its cue from the hypothalamus). This pair is as thick as thieves — when one of these organs sends a signal, it sets off a chain reaction in the other.

Because the HPA also controls the production of some of the most important hormones related to fertility, thyroid dysfunction can impact how much of those hormones are released. When any of these deviations from the status quo happen, they can disrupt (or stop) menstrual cycles. Without the complete cycle, including ovulation, fertilization, and implantation, you can’t get pregnant naturally.

As a result of these hormone changes, both hyperthyroidism and hypothyroidism have been linked to abnormal menstrual cycles. (Several studies point to prolactin as the main driver in this.)

All that said, getting a handle on your thyroid through treatment can reduce the chances of related fertility issues down the line. In one study of a group of almost 400 women suffering from infertility, 24% of participants were found to have hypothyroidism — but within a year of treatment, 76% were able to conceive. (The power of medicine!)

“Normal range” TSH levels and unexplained fertility problems

In one study conducted between 2000 and 2012, women with unexplained infertility were found to have TSH levels on the higher end of “normal” than the control population — which is indicative of subclinical (read: mild) thyroid disorder, though not of full-blown hypothyroidism.

What this means: Even less-dramatically elevated, within-range levels of TSH can result in difficulty conceiving. (All the more reason to check in with your levels and get ahead of any issues.)

What about thyroid dysfunction and male fertility?

An impaired thyroid doesn’t just have the power to impact women’s ability to conceive — there’s also an effect on male fertility. Why? Because T3 and T4 both play a role in the development and function of the testes.

Hyperthyroidism can lead to reductions in:

  • Semen volume, or how much is ejaculated
  • Sperm density, or the sperm count per milliliter of semen
  • Sperm motility, or how well and efficiently the sperm move
  • Sperm morphology, or the size and shape of sperm

Hypothyroidism can lead to reduction in:

  • Sperm morphology

Just like the effects of thyroid dysfunction on people with ovaries, people with testes can reverse thyroid-related fertility issues once the gland is back in working order after treatment.

Your thyroid while you’re pregnant

There are a few ways pregnancy can be impacted if thyroid problems aren’t addressed first.

Increasing demands from a developing baby can occasionally cause new onset or worsening hypothyroidism for pregnant women.

Additionally, impaired thyroid function (due to diagnosed conditions and/or the presence of thyroid antibodies) prior to and during conception can impact healthy brain development in the fetus. That said, T4 therapy through meds like levothyroxine can decrease the risk of babies born with a lower body weight and other complications.

As for pregnancy loss and miscarriage, there’s been little evidence found to directly link hyperthyroidism (in the absence of autoimmune thyroid disorder, or AITD) to either. But one study did find that the risk of miscarriage is doubled with hyperthyroid women as compared to women without thyroid issues. (This is likely due to excess thyroid hormones toxically affecting the development of embryos.) In terms of hypothyroidism (overt or sublinical), there is evidence that suggests that inadequate treatment can lead to infertility, miscarriage, and adverse pregnancy outcomes.

After pregnancy, there’s also an increased risk for postpartum thyroiditis, or inflammation of the thyroid — this happens to five to ten out of every 100 women within the first year after childbirth. This can lead to temporary (lasting up to a year) hypothyroidism, hyperthyroidism, or hyperthyroidism followed by hypothyroidism.

Treatments for thyroid disorders

Treating thyroid disorders before trying to have kids can decrease the risk for any issues impacting your ability to get pregnant or have a successful pregnancy. The treatments for hyperthyroidism and hypothyroidism vary depending on severity of symptoms and provider preference, but they can often be treated with medications prescribed by your doctor.

Typically, patients will be under close surveillance with initial treatment (labs drawn every six weeks to a few months) until a proper maintenance dose can be picked. Once a patient has a maintenance dose, thyroid labs will still need to be checked on a yearly basis.

So, what’s the bottom line with all this?

Well, for starters, even though the thyroid is a pretty tiny gland, it’s important. Because it isn’t always checked regularly, being aware of the symptoms of thyroid imbalance can be incredibly useful. If your periods are irregular or you’re experiencing something unusual that resembles any of the above symptoms, pay a visit to your doctor. They can help you get to the bottom of it and treat the problem before it makes getting pregnant more difficult.

Last but not least: If all of this has got you thinking about your own thyroid levels, or your fertility as a whole, take the Modern Fertility test to complete a few pieces of the reproduction puzzle. Because even teeny glands like the thyroid and short, three-letter acronyms like TSH can give you a big-picture perspective of your health. (Talk about powerful.)

For more info, you can watch our Q&A with Dr. Cindy Duke below.

This article was medically reviewed by Dr. Eva Marie Luo, an OB-GYN at Beth Israel Deaconess Medical Center and a Health Policy and Management Fellow at Harvard Medical Faculty Physicians, the physicians organization affiliated with the Beth Israel-Lahey Health System.

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Sarah duRivage-Jacobs

Sarah duRivage-Jacobs is a writer and editor at Modern Fertility. She lives with her creamsicle cat, Jasper, in New York City and doesn't believe in the concept of TMI.

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