The results of thyroid blood tests offer information about how well or poorly a patient’s thyroid gland is functioning. Though general normal ranges have been established, these ranges tend to vary from laboratory to laboratory. The numbers that follow should be used as a guide and should not be considered definitive. Patients should consult their physician about the implications of their test results.

If a physician suspects a thyroid problem, a TSH (thyroid-stimulating hormone) test is usually the first test to be performed. Normal results for TSH testing differ depending on whether the patient has a history of thyroid problems.

Normal levels of TSH for patients with no history of thyroid problems are 0.4 to 4.0 milli-international units per liter (mIU/L). However, patients who have no history of a thyroid problem but who have a reading of 2.5 mIU/L or higher may develop hypothyroidism in the future and should be monitored closely. Normal levels for patients who do have a history of thyroid problems are 0.5 to 2.5 mIU/L.

TSH readings that are higher than normal may indicate any of the following:

• Congenital hypothyroidism
  
  
• Primary hypothyroidism
  
  
• TSH-dependent hyperthyroidism
  
  
• Thyroid hormone resistance
  
  
• Exposure to mice

TSH readings that are lower than normal may indicate any of the following:

• Hyperthyroidism
  
  
• TSH deficiency
  
  
• Use of certain medications (e.g., dopamine agonists, corticosteroids, somatostatin analogues, bexarotene)

A thyroxine (T4) test may be conducted in conjunction with the TSH test. Generally, normal results for T4 testing are between 4.5 to 11.2 micrograms per deciliter (mcg/dL). Higher than average T4 levels in addition to lower than average TSH levels may indicate a form of hyperthyroidism, such as:

• Graves’ disease
  
  
• Toxic multinodular goiter
  
  
• Subacute or chronic thyroiditis
  
  
• Early Hashimoto’s disease
  
  
• Iodine-induced hyperthyroidism
  
  
• Germ cell tumors (found mainly in the ovaries and testicles)
  
  
• Trophoblastic disease (type of uterine cancer)

Levels of T4 that are lower than normal may indicate hypothyroidism, or any of the following:

• Malnutrition
  
  
• Systemic (throughout the body) illness
  
  
• Use of certain medications (e.g., dexamethasone, propranolol, lithium, iodine, methimazole, propylthiouracil, interferon alfa, interleukin-2 and amiodarone)

A triiodothyronine (T3) test may be performed if abnormal results are reported in TSH and T4 testing. Generally, normal results for T3 testing are between 100 to 200 nanograms per deciliter (ng/dL). Greater than normal T3 levels in addition to low TSH levels may indicate hyperthyroidism. In rare cases, this also may indicate thyroid cancer. Tests results in which T3 levels are lower than normal may indicate hypothyroidism, chronic illness or starvation.

Results from the three main types of thyroid blood tests may be combined to arrive at a diagnosis. Such results are as follows:

Another test, called a serum TBG test, measures the level of a protein called thyroxine binding globulin (TBG) in the blood. Normal values for TBG testing differ depending on the method used to analyze a sample. When electrophoresis (the use of an electrical field to separate charged particles) is used, normal values range from 10 micrograms (mg)/100 milliliters (ml) to 25 mg/100 ml. When radioimmunoassay (procedure using small amounts of a radioactive substance) is used, normal levels range from 1.3 to 2.0 mg/100 ml.

Elevated levels of TBG are normal in newborns. However, in adults elevated levels of TBG may indicate hypothyroidism and other problems with the thyroid gland. It also may indicate:

• Pregnancy
  
  
• Liver disease
  
  
• Acute intermittent porphyria (an inherited disorder of metabolism)

TBG levels that are lower than normal may indicate hyperthyroidism or any of the following:

• Acromegaly (disorder caused by overactive pituitary gland)
  
  
• Acute illness
  
  
• Malnutrition
  
  
• Stress associated with surgery
  
  
• Nephrotic syndrome (a disorder, sometimes caused by diabetic nephropathy, that involves proteinuria, high cholesterol and edema)

In some cases, high or low levels of TBG are due to an inherited trait and are not signs of any disorder.