Hypothyroidism (underactive thyroid) is when the thyroid gland produces less thyroid hormone than it should. The thyroid gland produces two hormones: triiodothyronine (T3) and thyroxine (T4). These hormones play an important role in metabolism. The thyroid gland is controlled (regulated) by thyroid-stimulating hormone (TSH). TSH is produced by the pituitary gland, which is located in the brain.
Hypothyroidism can prevent the release of the egg from the ovary (ovulation). Typically, for women who have periods (menstruate) each month, an egg is released from the ovary each month. But women who have hypothyroidism may release an egg less frequently or not at all.
Hypothyroidism can also interfere with the development of an embryo (fertilized egg). This increases the risk of miscarriage. Also, if you are pregnant and your hypothyroidism is not treated, your baby may be born prematurely (before the predicted due date), weigh less than normal, and have lowered mental capacity.
The main test used to detect hypothyroidism is a blood test called TSH (thyroid stimulating hormone). An elevated TSH level usually means the thyroid gland is not making enough thyroid hormone, and the pituitary gland has responded by making more TSH to try to get the thyroid hormone levels where they should be.
Other blood tests include measuring T4 and thyroid autoantibodies. T4 is a hormone produced directly by the thyroid gland. It is typically low in patients with hypothyroidism. Sometimes the immune system can make antibodies against your own body—such as against your thyroid. Thyroid autoantibodies are seen in patients with Hashimoto’s (autoimmune) thyroiditis.
Prolactin is a hormone produced by your pituitary gland that causes breasts to grow and develop and causes milk to be made after a baby is born. Hyperprolactinemia is a condition of too much prolactin in the blood of women who are not pregnant and in men. Causes of hyperprolactinemia include pituitary tumors (prolactinomas), untreated hypothyroidism, and certain medications that antagonize dopamine. Hyperprolactinemia is relatively common in women. In the setting of hyperprolactinemia, a woman might have trouble getting pregnant or her breasts may start producing milk outside of pregnancy (galactorrhea). Ninety percent of women with galactorrhea also have hyperprolactinemia. High prolactin levels interfere with the normal production of other hormones, such as estrogen and progesterone. This can change or stop ovulation (the release of an egg from the ovary). It can also lead to irregular or missed periods.
The events of the menstrual cycle are controlled by an interplay of hormones secreted by three organs collectively known as the hypothalamic-pituitary- ovarian-axis, (H-P-O axis). The three organs are:
1. The hypothalamus at the base of the brain which secretes gonadotropic releasing hormone (GnRH)
2. The pituitary gland located below the hypothalamus which secretes follicle stimulating hormone (FSH) and luteinizing hormone (LH) in response to GnRH
3. The ovary which secretes the steroid hormones estrogen and progesterone
These three organs communicate with each other through the production of their hormone signals. The hypothalamic causes of anovulation result from decreased or dysfunctional production of GnRH. Disruption of the hypothalamic signals may occur through stress (physical or emotional), rapid weight loss, eating disorders, certain medications, or brain tumors. Many times the source of the problem is elucidated by the medical history.