When a person has an underactive thyroid, or hypothyroidism, her thyroid does not produce enough of the thyroid hormones — triiodothyronine (T-3) and thyroxine (T-4) — which are involved in metabolism. While both men and women can have an underactive thyroid, women who are over age 50 are more likely to have the condition, according to the MayoClinic.com.
Several conditions can cause an underactive thyroid. MedlinePlus noted that the most common cause is an inflammation of the thyroid gland. When the thyroid becomes inflamed, its cells become damaged.
The autoimmune disorder, known as Hashimoto’s disease, may cause hypothyroidism, as the immune system mistakes the patient’s thyroid gland as a foreign object and attacks it. About 0.1 to 5 percent of adults in Western countries have this form of chronic thyroiditis, according to MedlinePlus.
Some medications may cause hypothyroidism. Medications that treat hyperthyroidism, or an overactive thyroid, such as methimazole or propylthiouracil, may cause hypothyroidism.
Lithium, a psychiatric medication used to treat disorders such as bipolar disorder, may affect thyroid function. Patients who take amiodarone may develop hypothyroidism.
Other forms of treatment may cause an underactive thyroid, such as radiation therapy or thyroid surgery. Patients who suspect their medications have contributed to an underactive thyroid should talk to their doctors and should not alter their medication unless under the direction of a doctor.
After pregnancy, some women may develop hypothyroidism. If the symptoms of hypothyroidism occur after a women became pregnant, it is called postpartum thyroiditis. Sheehan syndrome may also cause an underactive thyroid, noted MedlinePlus.
With Sheehan syndrome, a woman bleeds severely either during her pregnancy or during childbirth. This severe bleeding damages the pituitary gland, affecting its normal functioning. The pituitary gland is involved in thyroid gland function, so damage to the pituitary gland affects thyroid function.
A congenital defect may affect normal thyroid function.