甲状腺激素（英语：thyroid hormones）是由甲状腺滤泡上皮细胞合成的酪氨酸碘化物。主要是四碘甲腺原氨酸（又名甲状腺素，缩写为T₄）和三碘甲腺原氨酸（缩写为T₃），此外，还有少量逆-三碘甲腺原氨酸（缩写为rT₃）。注意，甲状腺分泌的激素除了甲状腺激素外，还有降钙素。由于降钙素是甲状腺滤泡旁细胞产生的，所以，不属于甲状腺激素的范畴。

T₄和T₃均有生理活性，区别在于作用时间和强度：T₄活性低、起效较慢，但持续时间长；T₃活性高、起效快，但持续时间短。体内研究显示，在细胞水平发挥生理作用的主要是T₃；绝大多数T₄需转化为T₃之后才能发挥生理效应，从这个意义上讲，T₄更像是一种前激素。

The thyroid hormones, triiodothyronine (T₃) and its prohormone, thyroxine (T₄), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism. T₃ and T₄ are partially composed of iodine (see molecular model). A deficiency of iodine leads to decreased production of T₃ and T₄, enlarges the thyroid tissue and will cause the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T₄), which has a longer half-life than T₃. In humans, the ratio of T₄ to T₃ released into the blood is roughly 20 to 1. T₄ is converted to the active T₃ (three to four times more potent than T₄) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T₁a) and thyronamine (T₀a). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T₃ production.