An endogenous, tissue-specific molecule that functions to inhibit mitosis (cell division) in the very tissue releasing the molecule.

Specifically, thyroid colloid. Refers to the protein-rich fluid within the lumen of thyroid follicles. The major protein component of the thyroid colloid is thyroglobulin.

The enzyme found in target tissues of the thyroid hormones that converts thyroxine (also known as tetraiodothyronine or T₄) to triiodothyronine (T₃).

Specifically, 5'-deiodinase converts T₄ to the active form of the thyroid hormone, T₃.

5-Deiodinase converts T₄ to the inactive form of the thyroid hormone, reverse T₃ (rT₃).