About the test
T3 is a thyroid hormone, 20% of which, is produced by the thyroid gland and 80% in peripheral tissues by conversion of T4 to T3 (deiodination).
It circulates in blood either bound to proteins (total T3) or free of proteins (free T3).
T3 measurement is used for the diagnosis of hyperthyroidism and T3 thyrotoxicosis.
Sample
Venous blood
Preparation
None required.
In case the examinee is under thyroid medication he/she should postpone intake of medication until after the test.
Procedure
Venipuncture
Increased values
- Hyperthyroidism
- Graves’ disease
- T3 thyrotoxicosis
- Pregnancy
- Drugs (Estrogens, lithium, oral contraceptives, rifampicin, clofibrate)
Decreased values
- Thyroidectomy
- Anorexia nervosa
- Preeclampsia
- Liver cirrhosis
Additional information
- Low Τ3 levels may be indicative of hypothyroidism, however, they are not diagnostic since T3 is usually the last hormone affected in hypothyroid patients.
- Free T3 is measured with other hormones in case of hyperthyroidism.
- Free T3 plays a crucial role in central nervous system of infants.