Hypothyroidism - Diagnosis

Thyrotropin (Thyroid-Stimulating Hormone or TSH). Measuring TSH is the most sensitive indicator of hypothyroidism. (As with thyroxine levels, however, TSH levels can vary in pregnant women and patients who are ill with other conditions.) In general, results indicate the following:

TSH levels over 10mU/L. This is a clear indicator of hypothyroidism if T4 levels are low--and, in most cases, even if they are normal. Patients usually require thyroxine (T4) replacement therapy. They should also be tested for high cholesterol levels and antithyroid antibodies.
Levels between 4.5 mU/L and 10 mU/L. Patients with signs and symptoms of hypothyroidism, usually need thyroxine replacement therapy. Patients without symptoms have subclinical hypothyroidism and should be rechecked every 6 to 12 months. Antibody tests may also be performed.
TSH levels between 0.45 mU/L to 4.5 mU/L. These indicate normal thyroid function. (Abnormally low levels suggest hyperthyroidism.)
It should be noted that the specific TSH measurement--even if it is significantly higher than 10 mU/L--is not associated with the severity of the condition. This can be determined only by measuring thyroxine levels and evaluating the patient's symptoms.

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Antithyroid Antibodies. If TSH levels suggest hypothyroidism or subclinical hypothyroidism, then some doctors may choose to perform a blood test for specific antithyroid antibodies that act against a factor called thyroperoxidase (TPO). Tests may also be performed for antibodies to thyroglobulin. Results depend on the patient's condition:

Patients with confirmed hypothyroidism (TSH levels over 10 mU/L). Positive test results in such patients confirm the need for thyroxine replacement therapy. (Even if antibody results are negative, such patients usually require thyroxine replacement therapy). About 90% patients with Hashimoto's thyroiditis test positive for antibodies to thyroperoxidase and up to half have thyroglobulin antibodies.
In patients with subclinical hypothyroidism (TSH between 4.5 mU/L to 10 mU/L). If antibody levels are high, then thyroxine therapy is usually warranted, since it indicates an underlying autoimmunity condition that poses a high risk for later thyroid failure. If the tests are negative, but patients have thyroid-related problems (such as high cholesterol, female infertility) they should be monitored annually with hormone tests.

Of note, about 10% of the American population and 25% of women over 60 years old carry these antibodies and the majority of these women have no thyroid problems. Only about 0.5% have full-blown hypothyroidism and 10% have subclinical hypothyroidism. In one 10-year study, however, people with normal thyroid results and high levels of antibodies still had an annual risk of 2 - 4% for developing hypothyroidism.