How Does Your Thyroid Affect Fertility?

Your thyroid gland is a small butterfly-shaped organ that makes your body’s supply of thyroid hormones.  Located at the front of your neck, just above your collar bone, your thyroid gland controls your metabolism.

Your thyroid gland secretes the thyroid hormones, thyroxine (T4) and triiodothyronine (T3).

Your thyroid hormones have many jobs, including:

  • regulate appetite and absorption of glucose in the intestines (study)
  • impacts body weight and insulin sensitivity(study)
  • regulate heart rate and body temperature (study)
  • regulate oxygen consumption and mitochondrial activity(study)
  • are important in growth and development including brain development (study)

And… your thyroid hormone levels also play a big role in your ability to get pregnant and stay pregnant.


Hypothyroidism is a condition where your body either produces too little thyroid hormone or has problems converting the less active thyroid hormone (T4) to the more potent form (T3). Doctors categorize it as clinical or sub-clinical hypothyroidism, depending on the degree of impairment.

The American Association of Clinical Endocrinologists and the American Thyroid Association sets the guidelines and the clinical definitions of hypothyroidism in the U.S.

Their list of clinical symptoms includes:

  • Cognitive impairment (e. brain fog)
  • Diastolic hypertension (high bottom number in blood pressure)
  • Course facies (coarse facial features, or a puffy face)
  • Goiter (enlarged thyroid gland)
  • Hypothermia (lower body temperature)
  • Lateral eyebrow thinning (ends of the eyebrows are thin or non-existent)
  • Macroglossia – (enlarged tongue)
  • Periorbital edema (puffiness under the eyes)
  • Delayed relations phase of deep tendon reflexes (ankle reflex test)
  • Lab results that include elevated CRP, LDLc, triglycerides, and others
  • Heart electrocardiograph changes

Hypothyroidism and Trying To Conceive:

Research shows that women who are hypothyroid are at a higher risk for infertility. One study on infertility found that 24% of the women were hypothyroid, compared to a normal population rate of 2 – 4% hypothyroidism.(study) Other studies show similar results with 14-24% of infertile women having hypothyroidism.(study) (study)

The good news is that correcting the thyroid levels helps with conceiving!

A study of hypothyroid women who had previously been unable to conceive found that thyroid hormone medication restored fertility in over 76% of the women.(study)

Women with subclinical hypothyroidism also benefit from thyroid hormone treatment when trying to conceive. A study found that women treated with T4 thyroid medication for subclinical hypothyroidism had higher pregnancy rates and conceived more quickly. (study)

Thyroid hormone levels are also important for egg quality and in vitro fertilization. In a study of women with subclinical hypothyroidism who were undergoing IVF, the women who receive thyroid hormone treatment (T4 only) had higher quality embryos, lower miscarriage rate, and higher live birth rate than those who did not receive T4 treatment. (study)

Animal studies also show that long-term hypothyroidism reduces ovarian reserve. This may be one cause of premature ovarian insufficiency. (study)

PCOS and Hypothyroidism:

Polycystic ovarian syndrome (PCOS) and hypothyroidism often go hand-in-hand.

Women with PCOS often do not ovulate every month, thus leading to problems when trying to conceive. Hypothyroidism is also associated with infrequent menstruation and lack of ovulation. (study)

A recent meta-analysis combined the data from several studies on PCOS and hypothyroidism. It found that women with PCOS were at double the risk for subclinical hypothyroidism.  (study)

Male Fertility and Hypothyroidism:

Thyroid hormone levels are also important for men, of course!  Men with hypothyroidism have decreased sperm production and decreased sperm motility. This leads to problems with fertility. (study)(study)

Testing for Thyroid Problems:

The only way to truly know your thyroid status is to get a blood test done.  Most doctors include a TSH test in your annual blood work.

TSH (thyroid stimulating hormone) is produced by the pituitary gland. It acts as a signal to the thyroid gland, telling it to increase production of the thyroid hormones. Normal TSH levels range from 0.4 to 4.9 mU/L. Some practitioners use 4.0 as the top end of the normal range.

While TSH is the most common test performed, it often doesn’t reflect your actual thyroid status.  It just shows how much of the signal is being sent to the thyroid — not how much hormone the thyroid is producing.

Three other tests that are important for thyroid status are:

  • fT4: free T4 measures the amount of T4 available to enter the bloodstream.
  • fT3: free T3 measures the amount of unbound T3 available. T3 is the more potent thyroid hormone.
  • rT3: reverse T3 inactivates T3 to control the levels of fT3.

Other important tests both for thyroid function and in trying to conceive are thyroglobulin (TG) antibodies and thyroid peroxidase (TPO) antibodies test. Autoimmune thyroid diseases (Hashimoto’s and Grave’s) cause elevated TPO antibodies.

One significant cause of multiple miscarriages is also elevated TPO antibodies. Read more about that here.

If you don’t have health insurance or your doctor won’t order these tests, you can order them yourself online by clicking on the links below:

Genes Play a Role in Thyroid Function:

Both your environment and your genes influence your thyroid hormone levels.

Your TSH level, controlled by the pituitary gland, sends the signal to your thyroid gland to produce thyroid hormone. When your body’s levels of T3 and T4 are high, the pituitary sends less of the signal so your TSH levels drop. When your level of T3 and T4 are low, TSH levels will rise, telling the thyroid to produce more hormone.

Researchers think that your genetic variants control ~40% of your TSH level. (study)

One gene that modifies your TSH levels is the PDE8B gene. If you have your genetic data from 23andMe, AncestryDNA, etc., you can check to see how your PDE8B genetic variants are affecting your TSH levels.

Check your genetic data for rs4704397:

  • AA: increase in serum TSH (study)(study)
  • AG: increase in serum TSH
  • GG: no increase in serum TSH

Check your genetic data for rs6885099:

  • AA: increased serum TSH (study)
  • AG: increased serum TSH
  • GG: normal serum TSH

Your thyroid mainly produces T4, which is the inactive form of the hormone. Tissues throughout your body can then convert the T4 to T3, the active form.  This conversion from T4 to T3 uses the deiodinase enzyme, coded for by the DIO1 and DIO2 genes. These reactions need both iodine and selenium. (study)

Check your genetic data for rs2235544:

  • AA:  less conversion to free T3(study)(study)(study)
  • AC: somewhat increased fT3
  • CC: increased fT3 (study)

Check your 23andMe results for rs11206244 (v.4, v.5):

  • TT: higher rT3, lower free T3, higher fT4  (study)(study)
  • CT: lower free T3
  • CC: normal

Check your 23andMe results for rs225014 (v.4):

  • CC: decreased T4 to T3 conversion(study)
  • CT: decreased T4 to T3 conversion
  • TT: normal DIO2 enzyme, less likely to get Hashimoto’s(study)

What can you do to support thyroid health?

The foods you eat support your thyroid function through essential nutrients and minerals.  It is important to get enough vitamin C, zinc, selenium, iron, iodine, and magnesium.

Eating a healthy diet of whole, unprocessed foods are essential to both overall health and thyroid health.

Getting enough overall calories is important for thyroid health. If you have been on a low calorie or low fat diet for a while and have noticed an increase in hypothyroid symptoms, you may want to investigate whether you need to add more nutrient-dense foods into your diet. (study)


Zinc is vital to good thyroid health, and studies show that it is often low in women with hypothyroidism.  Several studies have shown that supplementing zinc increased serum free T3 levels in women with subclinical hypothyroidism. (study) (study) (study)


Case reports and studies from a fertility clinic show that most women with hypothyroidism are also low in magnesium.  The study also found that pregnancies increased as magnesium levels increase. (study)(study) (study)


Vital for thyroid hormone synthesis, your body requires small amounts of iodine. The European Food Safety Authority sets the recommended daily amount at 150mcg/day. When iodine levels are low (less than 50 mcg/day), a goiter may develop. (study)

Seaweed is a very rich source of iodine. Wakame, which is used in miso soup, and nori, used in sushi, are the most commonly eaten seaweeds. Seafood and fish are also good sources of iodine. In many countries, table salt has iodine added to it to prevent goiters.


Selenium is another trace mineral that is integral to thyroid hormone production. Selenium is so important that your body prioritizes its use to the thyroid; deficiencies in selenium usually affect thyroid function last.

In autoimmune thyroid disease, clinical trials have shown that selenium helps to reduce TPO antibody levels. In fact, a study showed that for a third of the patients, selenium alone was able to reverse subclinical hypothyroidism.(study)(study)(study)

Nigella sativa:

Black cumin seed, also called Nigella sativa, has been shown in animal studies to both increase serum T4 and increase the number of offspring. (study)

Studies of women with Hashimoto’s found that powdered black cumin seed reduced BMI and cholesterol when compared with a placebo.  (study)


In addition to producing TSH, your pituitary gland also produces melatonin at night. TSH levels actually regulate some of the key enzymes needed to make melatonin. And melatonin, in turn, increases some of the key players in thyroid hormone production.

In addition to being important for thyroid function, melatonin is also essential for egg quality and fertility.

Prioritizing good sleep and blocking out blue light from your TV, laptop, and phone at night can help raise melatonin levels.

For more about melatonin, read this article.

Eliminating toxins that harm your thyroid:

Eliminating thyroid-harming toxins is just as important as thyroid supporting nutrients.

Studies show that exposure to commonly used pesticides decreases free T4.  This is a large problem for both farm workers and for people living in agricultural areas where pesticides may leech into the water supply. (study)(study)

Regardless of where you live, one way to decrease your exposure to pesticides is to choose organic foods whenever possible. This is important both when trying to conceive as well as during pregnancy. Studies show that organochloride pesticide exposure during pregnancy alters both the mother’s thyroid hormones and can affect the fetus. (study)

Bromide is also harmful to your thyroid gland.  Structurally, bromide is like iodine and can be absorbed into the thyroid in place of iodine. This can eventually lead to a decrease in thyroid function.(study)(study)(study)

Sources of bromide in your environment include fabrics and furniture sprayed with flame retardant and hot tubs containing brominated chemicals.

Important Takeaways: 

  • Thyroid health is essential for a healthy pregnancy.
  • If you have symptoms of hypothyroidism, go beyond just testing TSH. Include free T4, free T3, reverse T3, and TPO antibody blood tests to truly know your thyroid status.
  • Healthy foods and a healthy environment are vital to thyroid function. Getting enough magnesium, selenium, iodine, and zinc are important.
  • Avoid toxins that zap your thyroid.