Thyroid Physiology

Roderick Clifton-Bligh, MB, BS, PhD ■ Leigh Delbridge, MD, FRCS

The thyroid gland contains two separate physiologic endocrine systems: one responsible for the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and the other responsible for the production of the hormone calcitonin.

The functional unit for thyroid hormone production is the thyroid follicle. This is composed of a single layer of cuboidal follicular cells surrounding a central space filled with colloid. The average size of a follicle varies from 100 to 300 |j,m, each of which is surrounded by a network of capillaries. The primary function of the thyroid follicle is to make and store thyroid hormones.

Calcitonin is produced by C cells within the thyroid. These cells, of neural crest origin, lie in a parafollicular position in direct contact with the follicular basement membrane.

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