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Understanding the Thyroid/Fertility Connection

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a blog by guest blogger Breea Johnson, MS RD LDN, Pulling Down the Moon Nutritionist

When someone is dealing with infertility, they often scour the world for answers. Many women never figure out why they have unexplained infertility or experienced a miscarriage. But for others, the answer may lie in a simple blood test to measure thyroid function as low thyroid function and thyroid autoimmunity are both associated with infertility and miscarriage.

Consider Ann*, a 32-year old, normal weight woman who had two miscarriages with no explanation. After going to a miscarriage specialist, she was found to have hypothyroidism (low thyroid function) and with thyroid medication was successfully able to get pregnant and carry the baby to full term.

The thyroid gland, one of the largest endocrine glands in the body, is shaped like a butterfly and is located on the front of the neck below the Adam’s apple. While it is very small (only 2 inches across), it affects our entire body, as the hormones it produces and secretes regulate metabolism, growth and development.

A well-functioning thyroid gland is pivotal for a healthy body and to conceive a healthy child. There is a direct interaction between the thyroid gland and the reproductive organs (the testes in men and ovaries in women). Hypothyroidism (low thyroid function) influences ovarian function by decreasing levels of sex-hormone-binding-globulin (SHBG) and increasing the secretion of prolactin. Prolactin, the hormone which stimulates milk production for breastfeeding, also affects ovulation and menstrual cycles. Prolactin inhibits Follicle-Stimulating Hormone (FSH) and gonadotropin releasing hormone (GnRH). With high levels of prolactin, which can be caused by hypothyroidism, ovulation is not triggered and a woman cannot get pregnant. Additionally, in men, low FSH and GnRH caused by elevated levels of prolactin can prevent the maturation of sperm.

Another common disorder affecting fertility is Thyroid Autoimmune (TAI) disease (Graves Disease and Hashimoto’s Disease). It’s the most common autoimmune disorder and affects between 5-20% of women of childbearing age (Poppe, 2008). This group of disorders involves both the endocrine and immune systems. Interestingly, there is double the rate of miscarriages in TAI-positive women and TAI has also been found to be increased in women with endometriosis. In one study, researchers found a 75% reduction in miscarriage among TAI-positive women who received levothyroxine (a drug used to treat low thyroid function) during gestation and pregnancy. Subclinical hypothyroidism also may be an issue, as women with TAI show moderately high TSH (Thyroid-Stimulating Hormone) levels before pregnancy, although often within the normal reference range. There has been discussion among endocrinologists to move the limits of TSH (used to measure TAI) to 2.5mIU/l but that is not in effect yet.

We asked Dr. Christopher Sipe, a Reproductive Endocrinologist and one of our doctor partners at Fertility Centers of Illinois, whether he thinks thyroid disorders go undiagnosed early in the fertility journey when a woman is still with her OB.

“Because thyroid disorders are so common,” says Dr. Sipe “there are many women walking around who are undiagnosed. Thyroid hormone levels fluctuate in our daily lives and can be affected by diet, exercise, supplements, weight changes, medical conditions and seasonal changes. Therefore, a woman can be normal once, but 6-12 months later have an abnormal level. Pregnancy also causes a shift in the normal range of thyroid hormones, further making diagnosis of a thyroid disorder more difficult. It is for this reason that diagnosis pre-pregnancy is ideal and screening should be recommended to any woman considering pregnancy.”

Dr. Sipe recommends woman have her TSH checked when she is considering becoming pregnant, when she has an irregular menstrual cycle, if she has a family history of thyroid disorder or if she exhibits symptoms of a thyroid disorder. Symptoms of thyroid disorder include inability to lose weight, dry skin, dry hair, depression, fatigue, constipation or muscle aches and pains.

In the case of a woman who is trying to conceive, testing the thyroid early may save valuable time.

“Treatment can take three to six months before a steady hormone level is achieved, and then pregnancy can be attempted,” concludes Dr. Sipe.

*Name withheld for confidentiality reasons.

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