The thyroid gland secretes both of these. Thyroxine or T4 is the
most prevalent secretion of the thyroid - it is the inactive form
of tri-iodothyronine (T3) which is important for regulation of
metabolism. T3 and T4 are secreted by the follicular cells.
Calcitonin is secreted by the parafollicular cells of the thyroid
gland and acts to decrease blood calcium levels.