Hypothyroidism - Complications

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TSH (Thyroid-Stimulating Hormone). 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 need thyroxine (T4) replacement therapy. They should also be tested for high cholesterol levels and antithyroid antibodies.
• Levels between 4.5 - 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 - 12 months. Antibody tests may also be performed.
• TSH levels between 0.45 - 4.5 mU/L. These indicate normal thyroid function. (Abnormally low levels suggest hyperthyroidism, which is overactive thyroid.)
• 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, the doctor may choose to perform a blood test for specific antithyroid antibodies that act against a factor called thyroperoxidase (TPO). Tests can also check for antibodies to thyroglobulin. Results are particularly helpful in deciding how to treat someone with subclinical hypothyroidism.

About 10% of the American population and 25% of women over 60 years old carry these antibodies without having thyroid problems. Only about 0.5% have full-blown hypothyroidism, and 10% have subclinical hypothyroidism.

Other Hormone Tests for Thyroid Function. Other hormone tests may be performed if hyperthyroidism is suspected. They include tests for triiodothyronine (T3) and thyroglobulin (also called thyroid binding globulin). Such measurements also help detect sudden temporary increases in thyroid hormone (thyrotoxicosis) that can precede certain forms of autoimmune thyroiditis.