Which follicular cell is responsible for making up 95% of thyroid hormones?

Thyroxine, commonly referred to as T4, is produced by the thyroid follicular cells and constitutes the majority of the thyroid hormones synthesized by this gland, accounting for about 95% of its hormone output. This hormone plays a crucial role in regulating metabolism, growth, and development in the body.

T4 consists of four iodine atoms, which are essential for its activity, and is produced in greater quantities compared to triiodothyronine (T3), which is the more biologically active form of thyroid hormone. Although T3 has a significant role in metabolic processes, the thyroid primarily secretes T4, which can be converted into T3 in peripheral tissues as needed.

Calcitonin, on the other hand, is another hormone produced by the thyroid, specifically by parafollicular cells (C cells), but it is not responsible for the majority of metabolic regulation nor does it constitute a significant proportion of the thyroid hormone output. Thyroid-stimulating hormone (TSH) is produced by the pituitary gland and regulates the production of T3 and T4 in the thyroid but is not produced by the thyroid itself.