Why Would a Doctor Test Thyroid Hormone Levels?
Physicians may be interested in testing thyroid hormones for patients who present with symptoms typical of thyroid dysfunction, including overactive (hyper) or underactive (hypo) thyroid. Thyroid blood tests are also ordered when a thyroid goiter or an enlarged thyroid is felt on physical exam. The routine thyroid tests include T4 and TSH levels, but T3 levels are tested when a patient is suspected to have hyperthyroidism, but their T4 level is normal. Although T4 is normally elevated in hyperthyroidism, in some cases T4 is normal, while T3 is high, still resulting in hyperthyroidism. This is called T3 thyrotoxicosis, and it occurs when the thyroid produces excess T3, but normal levels of T4 (about 5% of hyperthyroid patients). Thus, for these patients, the only abnormal values may be triiodothyronine test results. In order to test hormone levels, blood will be drawn from the patient’s veins, likely from the arm or hand.
