Thyroid Health: Often-Neglected Foundation of Reproductive & Prenatal Health
[Last updated: January 10, 2023]
One important component of reproductive and prenatal health that's often neglected is the health of the thyroid glands. Thyroid glands produce hormones that touch almost all aspects of female reproductive processes - and drive the baby's brain development during early pregnancy.
Thyroid problems and insufficiency of iodine, the crucial ingredient of thyroid hormones, are surprisingly common among pregnant women. The good news is, however, that there are things you can do - on your own and with your doctor - to prioritize your thyroid health. Let's dive in.
Thyroid glands regulate metabolism via thyroid hormones
Thyroid glands, the butterfly-shaped organ found on the front side of your neck, produce two important hormones that regulate the body’s metabolism. On cue from the pituitary gland in the form of thyroid stimulating hormone (TSH), the thyroid glands release T3 (triiodothyronine) and T4 (thyroxine). T3, the active form, is 3 to 5 times as potent as T4, which is converted into T3 in tissues and organs as needed.
Thyroid hormones are best known for their roles in regulating metabolic rate, body temperature and heart rate. However, both T3 and T4 affect almost all of the physiological processes in the body. This is why symptoms of thyroid conditions include whole-body problems like unintended changes in body weight, fatigue, disruptions in sleep pattern, intolerance to heat or cold and so on, depending on whether the thyroid glands are underactive (hypothyroidism) or overactive (hyperthyroidism).
Thyroid hormones touch every aspect of women’s reproductive health
Importantly, T3 and T4 have been shown to influence many of the processes essential to women’s reproductive health:
- Reproductive hormones: T3 and T4 regulate the synthesis of many reproductive hormones, including prolactin and gonadotropin releasing hormone (GnRH), estradiol (E2), luteinizing hormone (LH) and progesterone, as well as sex hormone-binding globulin (SHBG).
- Oocyte (egg) maturation and ovarian reserve: Scientists have found thyroid hormone receptors on the surface of the ovaries, the eggs themselves and granulosa cells (the cells that surround the developing eggs and provide structural and nutritional support). The presence of thyroid hormone receptors suggests that thyroid hormones may have a role to play in the ovaries. While research is still developing and the results are conflicting, some retrospective, animal and in vitro studies have also found a possible correlation between normal thyroid function and healthy ovarian reserve.
- Menstrual cycles: Women with normal thyroid functions are more likely to have regular menstrual cycles (92%) than women with overactive thyroid (78%) or underactive thyroid (40-75%, depending on the study). Thyroid hormones’ impact on the production of reproductive hormones likely play a leading role here, as precisely timed release of multiple hormones like E2, progesterone, FSH and LH are necessary for regular menstrual cycles.
- Ovulation: Because reproductive hormones like FSH, LH and GnRH also control ovulation, healthy thyroid hormone levels may also be required for regular ovulation. For example, in the luteal phase, scientists have postulated that thyroid hormones support the sustained production of progesterone in the corpus luteum (the structure that forms in the follicle that has ovulated).
- Endometrium: The aforementioned rise in progesterone in the luteal phase is what triggers the thickening of the endometrium, in preparation for an embryo to implant. T3 and T4, hence, may also drive the embryo-welcoming development (and maintenance for 10-12 days) of the endometrium through their effects on the reproductive hormones that control endometrial thickening.
(On an important side note, thyroid hormones are essential for men’s reproductive health, too.)
Thyroid hormones support pregnancy & baby’s cognitive development
In addition to supporting reproductive health, normal thyroid function has been shown to promote the growth and development of the brain and cognitive functions in the baby during pregnancy and after birth. In the early days of pregnancy, the fetus is completely dependent on maternal thyroid function for the T4 they need until they have their own functional thyroid glands – making thyroid health particularly important during early pregnancy.
Given the key role of the mom's thyroid glands in the baby's brain development, you can think of iodine as another essential prenatal nutrient for baby's brain health, just like folate and choline.
During pregnancy, the thyroid glands use extra iodine to increase the production of T4-binding globulin (TBG) to increase the amount of T3 and T4 and meet this increased demand from the additional, growing life.
Iodine: the essential ingredient for healthy thyroid hormone levels
Iodine is the indispensable ingredient in the production of the thyroid hormones. Both T3 and T4 molecules contain iodine, a trace element belonging to the halogen family. (The numbers after the letter T refer to the number of iodine atoms in each hormone.) Without iodine, the body cannot synthesize these hormones, and that can lead to thyroid problems.
In the US, the daily recommended intake of iodine is 150 mcg for adult women (and men), but this increases to 220 mcg for pregnant women and 290 mcg for women who are nursing, to account for the increased need to support the baby’s need for T3 and T4. Iodine deficiency, which is the leading cause of developmental delay in the world, is rare in the US and other developed countries, partly thanks to the addition of iodine to table salt.
Pregnant women are at a higher risk of iodine deficiency
However, pregnant women are at a higher risk of iodine deficiency or insufficiency. WHO defines 150-249 mcg/L of urinary iodine as adequate for pregnant women. Multiple analyses of CDC’s NHANES (National Health and Nutrition Examination Survey) data found that a substantial portion of pregnant women in the US fall below this level – they may not have enough iodine in the body to synthesize enough thyroid hormones for both the mom and the baby.
Consider asking your doctor for a thyroid function test
Despite the prevalence of iodine insufficiency among pregnant women and the importance of healthy thyroid hormone levels in pregnancy, not all OBGYN doctors screen for thyroid issues. Universal thyroid screening as a part of routine prenatal care is still a controversial topic among experts, as some doctors worry that testing can lead to overtreatment of borderline cases.
However, according to one estimate, 55% of women with thyroid problems are not being diagnosed due to lack of universal screening, so it may be a good idea to get tested when you are trying or pregnant.
American Thyroid Association recommends testing for thyroid function for women who fit a relatively broad criteria, including age over 30; history of autoimmune diseases, pregnancy loss, preterm birth or infertility; more than 2 prior pregnancies; family history of thyroid problems and more. (See the link above for a full list.) If you fit any of the criteria, ask your doctor to test your thyroid function.
Include iodine-rich foods in your diet to support thyroid health
To support thyroid health, incorporate these iodine-rich foods in your diet:
- Dairy products like yogurt (116 mcg in 1 cup of Greek yogurt), milk (85 mcg/cup) and cheese (14 mcg/oz of cheddar cheese)
- Eggs (26 mcg)
- Seafood like cod (158 mcg/3-oz serving) and oysters (93 mcg/3-oz serving)
Iodine superstar foods: Seaweed
One iodine superstar food category is seaweed. Nori, the seaweed used to wrap sushi rolls, has over 200 mcg of iodine per 10 grams, while wakame, the seaweed you find floating around in your bowl of miso soup, has 280% of daily iodine need in just 2 tablespoons.
Iodized table salt
Another major source of iodine among people living in the US and many other developed countries is the iodized table salt. A quarter tablespoon delivers 76 mcg of iodine, about 35% of the daily need for pregnant women. If you use prefer other kinds of more flavor- and mineral-rich salts like sea salt, Himalayan rock salt or fleur de sel, however, you will have to look to other sources. These natural salts are not fortified with iodine.
Prenatal vitamins with iodine (caution: not all prenatal vitamins!)
Prenatal vitamins can also be a major source of iodine for pregnant and nursing women. In the US, inclusion of iodine in prenatal vitamins is not mandated. A 2017 survey found that about 40% of prenatal vitamins on the market, both prescription and over-the-counter, did not contain iodine.
So, it’s important to check the supplement fact panel and make sure that your prenatal vitamins does. According to the study, the median iodine levels of the prenatal vitamins that did contain iodine was 150 mcg, enough to reach the 220 mcg/day requirement, if combined with other iodine-rich foods.
Thyroid autoimmunity: When enough iodine isn’t the solution
It is important to note that many women, especially in developed countries where severe iodine deficiency is relatively uncommon, develop thyroid problems for another reason: thyroid autoimmunity. A common condition that affects about 10% of women, thyroid autoimmunity causes the body’s defense system to attack the thyroid glands and the thyroid hormones.
Women with thyroid autoimmunity are at higher risk of fertility struggles – and increasing iodine intake won’t address this kind of thyroid issues.
Your doctor can usually diagnose thyroid autoimmunity with blood tests for thyroid hormone levels and thyroid antibodies, as well as other tests like ultrasound imaging. If confirmed, there are effective medications and treatments to get your thyroid hormone levels back to normal. But the first step is to get the diagnosis - another reason it's important to discuss thyroid health with your doctor when you are preparing for a pregnancy, actively trying or pregnant.
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