The most important clue in diagnosing hypothyroidism is looking at possible symptoms and screening people who are vulnerable to subclinical hypothyroidism. Typically, people who have four or more of the symptoms of hypothyroidism outlined earlier should be screened. On the other hand, hypothyroidism is common enough so that TSH tests performed during a regular yearly checkup may detect hypothyroidism way before there are any discernible symptoms.
The next task in diagnosing hypothyroidism is to evaluate results of a TSH test properly. Abnormally high levels of TSH are a sign of hypothyroidism, while abnormally low levels of TSH are a sign of too much thyroid hormone, or thyrotoxicosis. It is important to note that the laboratory tests for people with symptoms of hypothyroidism should consist of both a free T4 level and a TSH level (see Chapter 2). This is because some people have trouble with their pituitary gland or brain that interferes with proper TSH production. In such situations, the free T4 level would be low while the TSH would be inappropriately normal or low.
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