Posted on April 12th, 2021by Dr. Taraneh Nazemin Women's Health

The Thyroid and Fertility: How They are Linked

So you recently went for a checkup with your doctor and they prescribed you thyroid medication but you don’t have any symptoms and you’re wondering why. Well here is everything you need to know about your thyroid, your fertility, and why you’re on medication.

The thyroid is a small, butterfly-shaped gland located in your neck that is essential to your body’s metabolism, development, and steroid production. It gets signals from the pituitary gland, located in the brain, via thyroid stimulating hormone (TSH). TSH tells your thyroid to produce thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4), which then circulate in your body. When your thyroid hormone production is low, called hypothyroidism, TSH rises to boost production. If you have an excess of thyroid hormone, or hyperthyroidism, TSH is suppressed.

How do you know if you have an overactive or underactive thyroid? Common symptoms associated with hypothyroidism are weight gain, fatigue, constipation, cold intolerance, hair loss, brittle nails, dry skin and menstrual irregularities. On the other hand, hyperthyroidism causes weight loss, palpitations, insomnia, sweating, heat intolerance as well as menstrual disturbances. Both hyper- and hypothyroidism are associated with infertility and pregnancy loss, and in some cases, women with either of these conditions may have no symptoms at all!

Thyroid testing is not necessary for all women, however women with infertility or a recent miscarriage, a strong family history of thyroid disease, or symptoms suggestive of a thyroid disorder should be evaluated. Initial testing with a TSH is usually sufficient, however if the TSH is abnormal then a T3 and T4 should be checked. In some cases, thyroid antibody testing (thyroglobulin antibodies or anti-thyroid peroxidase antibodies) may be warranted to determine if there is an autoimmune process that is causing abnormal thyroid production.

There are many different causes of hypothyroidism and hyperthyroidism. Autoimmune disease, called Hashimoto’s thyroiditis or Graves disease, infectious etiologies, or an iodine deficiency can all impact the thyroid. There is a subcategory of thyroid disease called “subclinical” hypothyroidism or hyperthyroidism, which is often asymptomatic and has a mild thyroid hormone profile. In subclinical hypothyroidism, for example, the TSH is elevated but T3 and T4 are within normal range. Women who present with subclinical thyroid disease are at risk of developing a full clinical hyper- or hypothyroidism and should be monitored.

While there is no clear evidence suggesting that subclinical hypothyroidism (SCH) is associated with infertility, an association between SCH and miscarriage has been identified. Studies have found that treatment of subclinical hypothyroidism can improve pregnancy rates and decrease miscarriage rates. It’s for this very reason that many women who are trying to conceive are given thyroid treatment in the absence of a true diagnosis of clinical hypothyroidism.

Treatment for hypothyroidism usually involves replacement of thyroid hormone with levothyroxine. Hyperthyroidism is treated with anti-thyroid medication, specifically methimazole or propylthiouracil. Thyroid levels are monitored every 6 to 8 weeks to determine efficacy of treatment so adjustments can be made accordingly. Women with subclinical disease may not need continued treatment after pregnancy but a consultation with a medical endocrinologist is important for long term management and care.

The thyroid also has an important role in pregnancy and fetal brain development. In fact, inadequate levels of thyroid hormone have been associated with impaired neuropsychological development of offspring. Throughout pregnancy, maternal T4 is transferred to the fetus but it is particularly essential in the first trimester when the fetal thyroid is not fully developed and cannot produce its own hormones. It is therefore very important that maternal thyroid levels are within normal range during this critical time.

There are several changes that the thyroid undergoes in pregnancy. The most important change is that there is an overall decrease in the amount of free circulating thyroid hormone in the body because there is a rise in thyroid binding globulin that attaches to free thyroid and minimizes activity. Most women with normal thyroid function have no problem with these shifts, but those with hypothyroidism usually need more thyroid hormone replacement to compensate. As a result, women with hypothyroidism need to increase their levothyroxine replacement about 30% in early pregnancy and monitor their TSH levels every trimester.

Hyperthyroidism can also impact pregnancy outcomes and can cause preterm delivery as well as fetal growth restriction. Medical treatment for hyperthyroidism may also need to be adjusted in pregnancy in order to minimize risks to the fetus. Propylthiouracil is usually preferred in the first trimester, whereas methimazole may be more favorable later in pregnancy and postpartum.

While the thyroid is a very little gland, it has a very big role in so many aspects of fertility and early pregnancy. Women should be aware of the symptoms of thyroid disease and seek an evaluation even in the absence of symptoms if they are struggling with infertility or a recent pregnancy loss. Luckily, thyroid treatment is most often fairly straightforward and can make all the difference in the world when it comes to reproductive planning.

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