A T3 test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor the status of a person with a known thyroid disorder. The T3 test is usually ordered following an abnormal TSH and T4 test. Either the total T3 or the free T3 may be ordered. Since most of the T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. T3 testing may be ordered along with thyroid antibodies to help diagnose Graves’ disease, an autoimmune disorder that is the most common cause of hyperthyroidism.
Diagnose a thyroid disorder in a person with symptoms
Screen newborns for an underactive thyroid
Monitor thyroid replacement therapy in people with hypothyroidism
Diagnose and monitor female infertility problems
Help evaluate the function of the pituitary gland (occasionally)
Screen adults for thyroid disorders, although expert opinions vary on who can benefit from screening and at what age to begin
Time course of changes in thyroid function tests in patients with thyroiditis. (T4= thyroxine; T3= triiodothyronine; TSH = thyroidstimulating hormone.)
At the hyperthyroid phage, the TSH is below the normal range while the T4 and T3 are above the normal range. After the treatment, the level of TSH goes up while the T4, T3 go down. When it comes to the hypothyroid phase, the TSH is higher than the normal range, and the T4/T3 are lower than the normal range. As the treatment continues, TSH, T4 and T3 go back to the normal range.