Diagnosing Hyperthyroidism: Overactivity of the Thyroid Gland

Part 3: Making the diagnosis of hyperthyroidism

Thyroid produces T3 and T4 in response to TSH.The actual diagnosis of hyperthyroidism is easy to make once its possibility is entertained. Accurate and widely available blood tests can confirm or rule out the diagnosis quite easily within a day or two. Levels of the thyroid hormones themselves, T4 and T3, are measured in blood, and one or both must be high for this diagnosis to be made.

It is also useful to measure the level of thyroid-stimulating hormone (TSH). This hormone is secreted from the pituitary gland (shown in orange) with the purpose of stimulating the thyroid to produce thyroid hormone. The pituitary constantly monitors our thyroid hormone levels, and if it senses the slightest excess of thyroid hormone in blood, it stops producing TSH. Consequently, a low blood TSH strongly suggests that the thyroid is overproducing hormone on its own.

Other special tests are occasionally use to distinguish among the various causes of hyperthyroidism. Because the thyroid gland normally takes up iodine in order to make thyroid hormones, measuring how much radioactive iodine or technetium is captured by the gland can be a very useful way to measure its function. The dose of radiation with these tests is very small and has no side effects. Such radioactive thyroid scan and uptake tests are often essential to know what treatment should be used in a patient with hyperthyroidism, and it's especially important if your doctor thinks your hyperthyroidism is caused by Graves' disease.

Common tests used to diagnose hyperthyroidism

  • Thyroid-stimulating hormone (TSH) produced by the pituitary will be decreased in hyperthyroidism.  Thus, the diagnosis of hyperthyroidism is nearly always associated with a low (suppressed) TSH level. If the TSH levels are not low, then other tests must be run.
  • Thyroid hormones themselves (T3, T4) will be increased. For a patient to have hyperthyroidism, they must have high thyroid hormone levels. Sometimes all of the different thyroid hormones are not high and only one or two of the different thyroid hormone measurements are high. This is not too common, as most people with hyperthyroidism will have all of their thyroid hormone measurements high (except TSH).
  • Iodine thyroid scan will show if the cause is a single nodule or the whole gland
  • We have a page that examines in detail all the laboratory and x-ray tests used to diagnose thyroid diseases, including a description of these tests and what they mean.

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