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

2. Parafollicular C cells  ->  Calcitonin

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