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Friday, July 31, 2015
When a patient is suffering from symptoms of a thyroid condition, there are a number of tests the doctor will perform to diagnose the type of thyroid condition a patient has.
Blood tests are used to diagnose thyroid disease. The levels of T4, T3 and "thyroid stimulating hormone" or "TSH" in the blood stream can be measured. An overactive thyroid would be expected to cause high levels of T4 and T3 and low levels of TSH.
An underactive thyroid would be expected to cause low levels of T4 and T3 and a high level of TSH. This is because the pituitary gland senses that there isn't enough thyroid hormone in the blood stream and is trying to stimulate the thyroid to make more.
Occasionally, a patient develops a disease in the pituitary gland which causes it to secrete too little TSH. Even though the thyroid gland itself is normal, it doesn't make enough thyroid hormone because it isn't being appropriately stimulated by the pituitary gland. This can result in hypothyroidism with a low T4 a low T3 and a low TSH.
Sometimes a nuclear medicine thyroid scan is ordered to help determine the cause of a patient's overactive thyroid. Patients swallow capsules that contain a harmless radioactive tracer. Some of this tracer winds up in the thyroid gland. Four, six or twenty-four hours after the capsule (depending on the preferences of the radiology department performing the scan) a scanner is placed over the thyroid gland. This scanner can take a picture of the thyroid gland by detecting the parts of the thyroid gland that have taken up the radioactive tracer.
In a normal thyroid gland, radioactive iodine is taken up to the same degree throughout the entire gland. If there is an area of the thyroid that does not take up radioactive iodine well, then it must be further investigated.
It can be useful to measure how much of the radioactive tracer wound up in the thyroid gland. This can help determine the cause of the overactive thyroid. However, the actual diagnosis of the overactive thyroid is made with blood tests, not with the scan.
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Last Reviewed: Mar 25, 2013
Thomas A Murphy, MD, FACP, FACE
Associate Professor of Medicine
School of Medicine
Case Western Reserve University