The main functions of the thyroid gland are brought about by the TWO main parenchymal cell types:

1. Follicular cells –>  Thyroid hormones T3 (tri-iodothyronine) and T4 (tetra-iodothyronine/thyroxine)

• Regulate basal metabolic rate (think about the clinical signs and symptoms of hyper / hypo thyroidism )
• Growth and development, especially of the central nervous system (read about Cretinism – hypothyroidism in infancy or childhood)
• The free (unbound) hormones are the metabolically active forms
• Learn more about T3 and T4 production:  http://fitsweb.uchc.edu/student/selectives/Luzietti/Thyroid_hormones.ht

2. Parafollicular C cells  –>  Calcitonin

• Calcium metabolism – maintains calcium homeostasis (generally, calcitonin opposes the effects of Parathormone)

What controls the amounts of T3 and T4 produced? As an endocrine organ, the thyroid gland is subject to the secondary (pituitary – TSH) and tertiary (hypothalamus – TRH) control mechanisms that all endocrine organs are subject to. This is the : Hypothalamo-pituitary axis