The thyroid gland and fertility
It is a known fact that to get pregnant and carry the pregnancy to term, it is important to have normal levels of thyroid hormones.
The hormones that the thyroid produces are called triiodothyronine (T3) and thyroxine (T4). In Assisted Reproduction, to test the thyroid we perform a Thyroid Stimulating Hormone (TSH) test in the blood. TSH is produced when the hypothalamus releases Thyroid Releasing Hormone and then triggers the pituitary gland to release TSH. TSH stimulates the thyroid gland and then produces T3 and T4. With a TSH test we find out if the thyroid is underactive or overactive. When thyroid function is low, the brain detects this and raises the TSH to stimulate the thyroid. A higher than normal TSH suggests that your body is detecting hypothyroidism. If we have hyperthyroidism, the body functions too fast. If we have hypothyroidism, the body slows down causing fatigue, loss of periods, weight gain, dry skin, etc.
The most common case in the spectrum of thyroid disorders in fertility is hypothyroidism, and this is most frequently caused by an autoimmune disorder. It is called Hashimoto’s thyroiditis and it occurs when the immune system attacks the thyroid gland with antibodies. Normal levels of TSH are between 0.5 and 5. When above 5 we say that the person has hypothyroidism. Many times the patient has no symptoms and this is called subclinical hypothyroidism. In the past 5 years in fertility, we have begun to treat subclinical hypothyroidism. In fertility the TSH level is thought to be safest if 2.5 or under. Most specialists use these parameters but there is an open debate in fertility about the effect of high TSH levels and at what level adverse effects are significant.
The baby depends on its mother’s thyroid gland until about 20 weeks of the pregnancy. Since the mother supplies the thyroid hormones needed by the baby, if she starts the pregnancy with hypothyroidism, her levels will rise. Many times, if hypothyroidism is detected before pregnancy, synthetic T4 is prescribed. When pregnancy is achieved, the endocrinologist keeps a close watch on the mother’s hormone levels and almost always increases her medication to cover the needs of both her and the baby.
The important question is what happens to a woman’s fertility if her thyroid is underactive? In subclinical hypothyroidism there are usually no symptoms. Once pregnant, women with hypothyroidism may have a higher incidence implantation failure and likelihood of miscarriage. These are grave side effects and this is the reason we test the thyroid before pregnancy.