Exams and Tests

Characteristic symptoms and physical signs can point hyperthyroidism; however, laboratory evaluation is necessary to establish the diagnosis and cause of hyperthyroidism.

Diagnostic lab tests performed on a blood sample include:

• Thyroid Stimulating Hormone (TSH)
  
  
  • TSH level will be low in hyperthyroidism
    
    
  • TSH assay is the most sensitive test for diagnosis of hyperthyroidism
  
  
  
• Free T4 (free thyroxine)
  
  
  • The active thyroid hormone in the blood will be high
    
    
  • In patients with unstable thyroid states, T4 levels are more accurate than TSH as indicators of thyroid status
    
    
  • With mild hyperthyroidism, the free T4 will remain high in the normal range.
    
• Triiodothyronine (T3) radioimmunoassay (RIA) or free T3
  
  
  • T3 is often elevated to higher level in severe hyperthyroidism.
  
  
  
• Thyroxine (T4)
  
  
• Thyroid autoantibodies: TSH receptor antibodies (TRAb) or Thyroid-stimulating immunoglobulins (TSI)
  
  
  • These antibodies are present in over half of patients with Graves's disease and stimulate the thyroid to grow in size.
    

If lab tests indicate hyperthyroidism, imaging tests may be used to further determine the cause.

• Radioactive iodine thyroid scan-with either 123I or 99mTc. In this test if you have a thyroid scan, you will be asked to take a drink, or swallow a pill containing radioactive iodine. You will then wait for the iodine to be taken up by the thyroid gland, and images will be taken to show the amount of iodine taken up by the thyroid.
  
  
  • Helps to determine the cause of hyperthyroidism and to assess whether any thyroid lumps or nodules are actively producing thyroid hormone
    
    
  • Increased uptake of iodine will be seen in a generalized pattern in Graves disease (See Figure 1 below), and in a localized pattern in toxic nodular goiter (See Figure 2 below)
    
    
  • Decreased uptake of iodine will be seen in subacute thyroiditis (See Figure 3 below)

Figure 1 Above: Radioactive iodine thyroid scan. Comparison between scans from the normal patient and a patient with Grave's disease. Note the overall increased uptake throughout the enlarged thyroid gland in the Grave's patient.

Figure 2 Above: Radioactive iodine scan of patient with toxic multinodular goiter. Note the patchy and darker appearance of the nodules that are producing excess amounts of thyroid hormone. Compare this to the normal scan shown in Figure 1.

Figure 3 Above: Radioactive iodine scan of a patient with subacute thyroiditis during the hyperthyroid phase. Note that there is very little iodine uptake in the thyroid. This is due to inflammation of the thyroid causing release of stored thyroid hormone (causing elevated thyroid hormone levels in the blood) and reduced iodine uptake.

• False Positive tests: high total T4 and total T3 levels or suppressed TSH levels
  
  
  • Estrogen administration or pregnancy can raise levels of TBG (thyroxine-binding globulin), resulting in high total T4 and total T3 levels, but normal free T4 and free T3 estimates and normal result on sensitive TSH assay
    
    
  • Euthyroid hyperthyroxinemia (another condition in which thyroid hormone levels appear to be elevated without an excess function of thyroid hormones) may also be attributable tan inherited condition of other abnormal binding proteins-albumin and prealbumin
    
    
  • Thyroid hormone resistance states
    
    
    • Increased serum T4 levels without hyperthyroidism, usually from an inherited condition.
      
      
  • Administration of corticosteroids, severe illness, pituitary dysfunction
    
    
    • These conditions may suppress the TSH level in the absence of hyperthyroidism

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