T3
This test measures the level of triiodothyronine (T3) in your blood. T3 is one of two major hormones made by your thyroid, a small, butterfly-shaped gland located near the throat. The other hormone is called thyroxine (T4.) T3 and T4 work together to regulate how your body uses energy. These hormones also play an important role in controlling your weight, body temperature, muscle strength, and nervous system.
The T3 hormone comes in two forms:
Bound T3, which attaches to protein
Free T3, which does not attach to anything
A test that measures both bound and free T3 is called a total T3 test. Another test called free T3 just measures free T3. Either test may be used to check T3 levels. If T3 levels are not normal, it can be a sign of thyroid disease.