T3 (triiodothyronine) is the active form of thyroid hormones. Some of it is produced by the thyroid gland, but more than 80% of it comes from the transformation of the T4 hormone into T3 in the tissues. Thyroid function is mainly assessed by measuring TSH levels. When TSH levels are low, laboratories first add a free-T4 test to confirm hyperthyroidism. More rarely, when low TSH levels cannot be explained by high levels of free T4 (see those headings), a measurement of free T3 levels is required.
High levels of free T3 usually accompany free T4 levels in hyperthyroidism or overtreatment with thyroxine (Synthroid©, Levothyronine). The level is sometimes the only one to rise in certain forms of hyperthyroidism (toxic adenoma, multinodular goitre). High T3, low TSH and normal T4 levels can also be seen when hypothyroidism is overtreated directly with T3 (liothyronine, Cytomel©) rather than with T4.
Low levels of T3 most often reflect T4 levels and are therefore of less clinical interest. T3 levels (with normal T4) are sometimes seen in certain diseases and chronic conditions that do not affect the thyroid.