Temporal Changes in Thyroid Test Results

For your consideration:

  1. How would you expect the thyroid tests (TSH and T4) to change over time after the thyroid gland was removed in our previous Grave's Disease patient (assuming she was placed in thyroid replacement therapy shortly after surgery)?

  2. What test would be most appropriate to use to follow her replacement therapy?

  3. How would her time course differ from a hypothyroid patient who was begun on thyroid replacement therapy without a preceding hyperthyroid period?

  4. In patients who were previously euthyroid, what effects does non-thyroidal illness have on thyroid tests?

Click on the shadowed text below to display time courses of test values. The thin red horizontal lines indicate reference ranges. Make sure to read the comments below the graph, as well.

Note that:

  1. T3 is a relatively sensitive indicator for hyperthyroidism (it goes up early), but a relatively poor indicator for hypothyroidism (it may never go below the reference range). T3 levels tend to have a fair amount of variability because the peripheral conversion of T4 to T3 is affected by illness and drug therapy. Generally, T3 doesn't add much information to that already available from TSH and free T4. Rarely, however, a thyroid gland may release primarily T3 rather than T4, leading to clinical thyrotoxicosis and TSH suppression without T4 elevation. This condition is called "T3 Toxicosis."

  2. TSH levels change relatively slowly after either thyroid removal (in hyperthyroidism) or initiation of thyroid replacement therapy (in hypothyroidism). Several weeks or months may be required for TSH equilibration. During this time, free T4 assay is used to follow the patient's status (and adjust the dose of replacement hormone). After TSH equilibration, it again becomes the best indicator of the patient's ongoing thyroid status.

  3. TSH and T4 levels fluctuate during non-thyroidal illness and may appear borderline hypo- or hyperthyroid at different points in the time course of disease. It's best to try to stabilize the patient and take care of other active health problems before working up potential thyroid disorders. Note that in severely ill patients, persistently declining T4 may be a poor prognostic sign.
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    Last modified: 1/17/97; Author: J. Harrison