Hypothyroidism occurs when the thyroid gland is not producing as much thyroid hormone as it should. Hypothyroidism is common in women of reproductive age.

Depending on its severity, hypothyroidism may cause a variety of symptoms and may affect all body functions. In mild cases, there may be no symptoms at all. In more severe cases, the body’s metabolism slows, causing mental and physical sluggishness and a variety of other symptoms. In women of reproductive age, hypothyroidism can be a cause for failure to ovulate or failure to ovulate regularly. This, in turn, causes a woman’s periods to occur less frequently (oligomenorrhea) or to stop completely (amenorrhea).

Thyroid gland

A common cause of hypothyroidism is the presence of elevated levels of anti-thyroid antibodies. Failure of the pituitary gland to secrete a hormone (TSH or thyroid stimulating hormone) to stimulate the thyroid gland (secondary hypothyroidism) is a less common cause of hypothyroidism. Other causes include congenital defects, surgical removal of the thyroid gland, irradiation of the gland, or inflammatory conditions. In an otherwise healthy woman, hypothyroidism can be easily detected by assessing the levels of TSH in the blood. A high TSH level indicates hypothyroidism. A slightly elevated level indicates a very mild case. A higher number indicates more severe disease and is often accompanied by a reduction in the level of T3 or T4 in the blood.

Sub-clinical hypothyroidism:

An elevated TSH with a normal T3 and/or T4

Overt or clinical hypothyroidism:

An elevated TSH and a low T4 and/or T3

Hypothyroidism and ovulation problems

Hypothyroidism can cause anovulation (failure to ovulate) directly or by causing elevation in another hormone called prolactin. Prolactin is the hormone used by the body to assist with lactation (milk production) after delivery. If a woman’s prolactin is elevated, she may experience milk discharge from her breasts (galactorrhea).

Treatment with oral thyroid hormone supplements (Synthroid, Levoxyl) will often correct the hypothyroidism and result in a return to normal ovulation and regular menstrual cycles. If prolactin levels were also elevated, then treatment usually results in a return of this hormone to normal levels with cessation of the breast discharge.

Once the thyroid is corrected, and ovulation is occurring, fertility is usually very good, unless there are other independent factors.

Hypothyroidism and miscarriage

A 2002 study of women with both clinical and sub-clinical hypothyroidism in early pregnancy found that the miscarriage rate was higher in both groups and that treatment with thyroid hormone could reduce the miscarriage risk.

Miscarriage risk may also be higher in women with anti-thyroid antibodies.

Hypothyroidism in pregnancy

It is extremely important to diagnose and treat clinical hypothyroidism in early pregnancy. The developing fetus cannot make its own thyroid hormone until about the 10th week of gestation. During this time, it relies on thyroid hormone production from the mom. Severe hypothyroidism is pregnant women, particularly in the 1st trimester, is associated with a host of pregnancy complications including pre-eclampsia (high blood pressure of pregnancy), placental abruption (separation of the placenta from the wall of the uterus), preterm birth, low birth weight and even fetal death. The babies can suffer from neurologic impairment, mental retardation, and intellectual impairment during childhood.

Fortunately, because women with hypothyroidism don’t usually get pregnant, clinical hypothyroidism in pregnancy is very rare, maybe occurring in 1 in 5000 pregnancies.

Sub-clinical hypothyroidism and IVF

A study presented in 2007 did not find any difference in the chances for IVF success if women had untreated sub-clinical hypothyroidism at the beginning of their IVF cycle.

Sub-clinical hypothyroidism in pregnancy

If a woman has only a mild elevation in her TSH and her T4 and T3 levels are normal, there is disagreement about whether these pregnancies are at any higher risk. Sub-clinical hypothyroidism is a bit more common, occurring in 2-5% of all pregnancies. Some studies have suggested that the children born may have lower IQ scores or impaired psychomotor development. These studies, which were small and often didn’t distinguish between clinical and sub-clinical hypothyroidism are difficult to interpret.

Very recently, researchers published results of the largest study to date of the effects of sub-clinical hypothyroidism on pregnancy. They screened 25,756 pregnant women and found 404 (2.3%) met the criteria for the diagnosis of sub-clinical hypothyroidism. The pregnancies in these women were 3 times more likely to be complicated by placental abruption and twice as likely to have preterm birth. The proportion of infants admitted to the neonatal intensive care unit, as well as those who developed respiratory distress syndrome (a common problem in premature infants) was doubled.

It is not yet known whether treatment of sub-clinical hypothyroidism will reduce these risks though it seems reasonable to think for now that it would.