Lesson 5: Diagnosing Hyperthyroidism

Laboratory Tests

Reference Ranges All laboratory tests have a normal or reference range that is listed below the test results. This range represents levels seen in normal individuals. However, levels that fall within the reference range may not be normal for a given individual. Studies show that thyroid test results vary widely in the normal population. When we’re in good health, our thyroid function test results remain remarkably stable. Normally, the pituitary gland keeps our thyroid hormone levels constant by increasing or decreasing TSH. The current AACE guidelines recommend a reference range of 0.3-2.5 mu/L for TSH. While most people have a TSH level between 0.3-1.0 mu/L, some people optimally function with a TSH level of 2.4 mu/L.

In patients undergoing treatment for hyperthyroidism, having thyroid hormone levels that fall within the reference ranges merely means that the therapy is working. Patients will still have symptoms of hyperthyroidism or hypothyroidism if their thyroid hormone levels are too high or low. Thyroid hormone is extremely potent, and even small variations from one’s optimal levels can have profound effects.

Thyroid Function Tests The FT4, FT3, and TSH tests are the primary tests used to diagnose hyperthyroidism. To screen for hyperthyroidism, a TSH level is run. If it’s abnormal the FT4 and FT3 tests are ordered. In hyperthyroidism, either of the available thyroid hormones, FT4 and/or FT3, may be elevated. It’s important to run both tests because many patients with both GD and toxic multinodular goiter will have elevated FT3 levels and normal FT4 levels.

In hyperthyroidism, the TSH level is low, usually suppressed to less than .01 mu/L. If TSH is low and thyroid hormone levels are normal, the patient may have subclinical hyperthyroidism. However, TSH can be low or suppressed in other conditions and as a result of various medications. Because TSH test results can be misleading, especially in Graves’ disease and thyroiditis, TSH test results should not be used to monitor anti-thyroid drug therapy. Although a low TSH doesn’t necessarily mean that hyperthyroidism is present, a TSH result higher than 2.5 mud/L suggests hypothyroidism. The TSH level will often rise quickly when patients move into hypothyroidism, either as a result of treatment or spontaneously.

Thyroid Antibody Tests Once hyperthyroidism has been established, it’s important to find out the cause. If characteristic eye symptoms are present, some doctors will diagnose Graves’ disease. However, because eye symptoms related to excess thyroid hormone can appear in any of the hyperthyroid disorders it’s important to run tests for thyroglobulin and TPO antibodies to determine if the hyperthyroidism is autoimmune. If Graves’ disease is suspected, a test for stimulating TSH receptor antibodies should be ordered. These antibodies are also known as thyroid stimulating immunoglobulins or TSI.

Tests No Longer Recommended With the availability of Free T4 and Free T3 tests, several of the older thyroid function tests still in use are no longer recommended. Before sensitive tests for measuring FT4 and FT3 became widely available, older tests, such as T4, T3, T3 uptake and the FT4 index or FTI, were used to approximate, that is, get a rough idea of FT4 and FT3 levels.

Total T4 and Total T3 levels Total T4 and total T3 levels, the T4 and T3 tests, measure levels of free thyroid hormone and also the binding proteins that carry this hormone. In this form, thyroid hormone is linked to protein molecules and inactive. Once the free hormone is cleaved or split from the binding protein, it is available to react with the body’s cells. When binding protein levels are elevated, levels of T4 and T3 are elevated. Binding protein levels are elevated in pregnancy, in patients on estrogens and many other medications. For this reason, FT4 and FT3 levels are the best indicators of thyroid function.

The T3 uptake test and the FT4 index In the T3 uptake procedure, reagent grade T3 is added to the patient’s serum to determine the quantity of binding proteins available to carry T3. From this result and a total T4 result, the approximate amount of free T4 can be calculated. This result is the FT4 index, the FTI, and the FT7. Values of FTI do not correlate well with direct measurements of FT4, and the FT4 assay is considered more reliable.

Test Interferences Many medications interfere with thyroid function tests. These medications are listed in the assigned reading. In addition, a condition of non-thyroidal illness, including recovery from surgery, will lower thyroid hormone levels and TSH. For this reason, thyroid function tests are not recommended in hospitalized patients.

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