Total triiodothyronine (T₃) concentration in serum/plasma

What Is Triiodothyronine (T₃)?

Triiodothyronine (T₃) is a hormone produced by your thyroid gland. It plays a vital role in controlling how your body uses energy, affects growth and development, and helps maintain normal body temperature. While thyroxine (T₄) is often measured first, T₃ is also important because it has a stronger effect on cells throughout your body. Your body converts some T₄ into T₃ to use more directly.

Why Might You Need This Test?

Doctors may order a T₃ test if you have symptoms like fatigue, weight changes, or unexplained heart issues, or if you have thyroid-related conditions such as hypothyroidism or hyperthyroidism. It can also be part of a broader thyroid function evaluation, especially when T₄ levels are abnormal or when T₃ is known to be critical for your symptoms.

How Is the Test Performed?

A blood sample is taken from a vein in your arm. The sample is sent to a lab where it is analyzed using an immunoassay method to measure the total T₃ concentration in your serum or plasma.

What Do Results Mean?

Results alone may not give a full picture without considering other thyroid-related tests (like T₄ or thyroid-stimulating hormone, or TSH). Abnormal levels can indicate thyroid dysfunction, but they often require additional context from your symptoms and other tests.