What is the Hypothalamic-Pituitary-Thyroidal (HPT) Axis?
Thyroid hormones are tightly regulated in the body. There is a feedback loop that keeps hormones at appropriate levels as depicted in the diagram below.
The hypothalamus and pituitary release just the right amount of hormones that stimulate the thyroid gland to release thyroxine (T4) and triiodothyronine (T3). The thyroid gland releases mostly thyroxine which gets converted into the active form of thyroid hormone.
There are thyroid hormone receptors or sensors on the hypothalamus and pituitary that then downregulate the thyroid from releasing too much hormone.