Many symptoms of hypothyroidism can occur in other diseases, so hypothyroidism usually cannot be diagnosed based on symptoms alone. Health care providers take a medical history and perform a thorough physical examination. Providers may then use several tests to confirm a diagnosis of hypothyroidism and find its cause.
Thyroid-stimulating Hormone (TSH) Test
The ultrasensitive TSH test is usually the first test a doctor performs. This test is the most accurate measure of thyroid activity available.
The TSH test is based on the way TSH and thyroid hormone work together. The pituitary gland boosts TSH production when the thyroid is not making enough thyroid hormone; the thyroid normally responds to TSH by making more hormone. Then, when the body has enough thyroid hormone circulating in the blood, TSH output drops. In people who produce too little thyroid hormone, the pituitary makes TSH continuously, trying to get the thyroid to produce more thyroid hormone.
Generally, a TSH reading above normal means a person has hypothyroidism and a reading below normal means a person has hyperthyroidism.
Other Tests
Health care providers may conduct additional tests to help confirm the diagnosis or determine the cause of hypothyroidism.
The T4 test measures the actual amount of circulating thyroid hormone in the blood. In hypothyroidism, the level of T4 in the blood is lower than normal.
The thyroid autoantibody test looks for the presence of thyroid autoantibodies. Most people with Hashimoto's disease have these antibodies, but people whose hypothyroidism is caused by other conditions do not.
Further Reading
Source: National Endocrine and Metabolic Diseases Information Service