The thyroid gland secretes three hormones:

1. 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.

2. 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.

3. 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.

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