Thyroid hormones

Collective term for the classical (bioactive) thyroid hormones produced in the follicular epithelial cells of the thyroid gland (thyreocytes):

• thyroxine (T4)
• triiodothyronine (T3, liothyronine).

Thyroid hormones play an important role in energy metabolism and the growth of individual cells and the entire organism. They are absolutely essential for life.

Besides T4 and T3, there are other iodothyronines (non-classical thyroid hormones) with weaker or missing bioactivity or with partly antagonistic effects.

• Thyronamines are hormones that have the opposite effects to the classical thyroid hormones. There is increasing evidence that they are formed from iodothyronines.
• Iodothyroacetates are deaminated iodothyronines which occur in serum in higher concentrations than T3 and have partial agonistic effects to iodothyronines.

Calcitonin, which is also produced in the thyroid gland (produced by the parafollicular cells, C-cells of the thyroid) is not called thyroid hormone, as it is neither structurally nor functionally related to the classical thyroid hormones.