A TSH test measures the amount of thyroid-stimulating hormone (TSH) in the bloodstream. TSH is the hormone that directs the thyroid to produce thyroid hormones. The test is used to diagnose thyroid disorders, and a high TSH level is usually an indicator of hypothyroidism.
Hypothyroidism occurs when the thyroid gland is underactive, causing the body to receive too little thyroid hormone.
Hypothyroidism can cause high TSH levels because the pituitary gland recognizes the body’s insufficient thyroid hormone levels and produces more TSH in order to stimulate hormone production.
Hypothyroidism is most often caused by an autoimmune disease that causes the body to attack its own tissues and hamper the thyroid’s hormone production. It can also result from thyroid surgery, medication to treat an overactive thyroid, radiation to the head and neck, a pituitary disorder, pregnancy or an iodine deficiency.
Because thyroid hormones are important for so many bodily processes, untreated hypothyroidism can cause heart disease, depression, reduced cognitive functioning, infertility, birth defects and myxedema, according to the Mayo Clinic.
High TSH levels can also indicate thyroid hormone resistance, a condition in which the body does not respond normally to thyroid hormones, says DrLowe.com.