Because hypothyroidism cannot be diagnosed on the basis of symptoms alone, healthcare providers use blood tests to confirm that a person has an underactive thyroid or to rule it out. The two most common tests used check the levels of certain thyroid hormones -- in particular TSH and T4. Another test looks for the presence of thyroid antibodies.
How Is Hypothyroidism Diagnosed?Many symptoms of hypothyroidism can occur in other diseases, so hypothyroidism usually cannot be diagnosed based on signs and symptoms alone.
Healthcare providers take a detailed medical history and perform a thorough physical examination. They may then use several tests to confirm a diagnosis of hypothyroidism and find its cause.
Tests Used to Diagnose HypothyroidismA couple of tests are used when diagnosing hypothyroidism, including blood tests for thyroid-stimulating hormone (TSH) levels.
Thyroid-Stimulating Hormone Test
The ultrasensitive TSH test is usually the first test a healthcare provider performs. This test is the most accurate measure of thyroid activity available because the levels can be elevated even with small decreases in thyroid function.
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.
However, in people who produce too little thyroid hormone, the pituitary makes TSH continuously, trying to get the thyroid to produce more thyroid hormone in response.
Generally, a TSH reading above normal means a person has hypothyroidism and a reading below normal means a person has hyperthyroidism.