The main hormones tested to determine thyroid function are T3, T4 and TSH. The first two measure the amount of hormone directly produced by the thyroid gland, the third test measures the amount of thyroid-stimulating hormone (TSH) produced by the pituitary gland. The results of one measurement alone will not result in a proper diagnosis; at least two of the three hormones should be measured, according to Endocrine Web.
T3 is the inactive thyroid hormone that the body can convert as needed to the active thyroid hormone. T4 is the active thyroid hormone. TSH is the hormone produced by the pituitary gland, the function of which is to stimulate the thyroid to produce T4.
Normal Test Results
The results represented here are the general normal limits, according to health-care-information.org. "T3: 80-220 ng/dl; T4: 5-13 ug/ml; TSH 0.55-3.89 ugIU/ml." Each lab or method of testing may have its own range for normal test results.
There are other thyroid hormone blood tests which may be used to differentiate between diagnoses if these basic thyroid tests, often referred to as the thyroid panel, indicate abnormalities. Additional testing may also be necessary to determine severity of the condition.
Thyroid Binding Globulin
Thyroid binding globulin (TBG) may be tested if the patient is without symptoms for thyroid disorder but the T3 or T4 levels are outside of the normal range. Since most of the thyroid hormones are bound to the TBG, if its levels are abnormal, it can cause false readings of the hormones themselves. It's possible that without a TBG test a patient can be falsely diagnosed as being hyperthyroid or hypothyroid.
The thyroid gland is the only structure in the body which produces thyroglobulin. Testing serum thyroglobulin levels is usually done in people who have had partial or total removal of the thyroid gland. The test will show whether residual thyroid tissue remains, which may or may not be cancerous.
Merck's authors recommend screening TSH every five years in men ages 65 and older, and in women 35 and older. Annual screening in recommended for people over age 70. Merck advocates that such screening is as cost effective as screening for other chronic diseases such as high cholesterol and breast cancer.