The thyroid gland produces and excretes two groups of hormones: thyroid hormones and calcitonin.
Thyroid stimulating hormone (TSH) is secreted from the anterior pituitary gland and is responsible for the synthesis and secretion of thyroid hormones. The secretion of TSH is in turn controlled by two mechanisms:

  • A negative feedback loop from plasma levels of thyroid hormones
  • Thyrotropin releasing hormone (TRH) secreted from the hypothalamus. This TRH level is also related to a negative feedback loop, forming the hypothalamic pituitary thyroid axis.

Hypothalamus to anterior pituitary is regulated by thyroid releasing hormone (TRH).

T3/4 inhibits the release of TSH and TRH.