The thyroid gland secretes three hormones:
- Thyroxine or 3:5,3':5' tetraiodothyronine (T4) is the major hormone secreted by the follicular cells of the thyroid gland. With 99.95% of the secreted T4 being protein bound, principally to thyroxinebinding globulin (TBG), to a lesser extent thyroxine binding prealbumin (TBPA) and albumin. T4 is involved in controlling the rate of metabolic processes in the body and influencing physical development.
- 3:5,3' triiodothyronine (T3) is 34 times more potent than T4. Only 15% of the total normal T3 concentration is directly secreted by the follicular cells. With 99.5% of the secreted T3 being bound to the same proteins as T4.
- Calcitonin is secreted by the Ccells of the thyroid gland. Its function is totally unrelated to the other thyroid hormones, since it's involved in calcium homoeostasis.
T4 (either total or free) along with thyroid stimulating hormone (TSH) are used as biochemical indicators of thyroid function,which aids the diagnosis and monitoring of either hyperthyroidism or hypothyroidism.
The homogenous latex agglutination in vitro diagnostic assay by the Bayer Corporation for the Technicon Immuno 1®System measures total T4 in serum by detecting the amount of agglutination.