Monday, December 5, 2022

Hypothalamic Pituitary Thyroid Axis (regulation, TRH, TSH, thyroid hormones T3 and T4)


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“The hypothalamic–pituitary–thyroid axis plays the major role in regulating thyroid hormone homeostasis in the body. The thyroid hormone are very important in fetal development and metabolism in adults. Important structures are the hypothalamus, pituitary gland specifically the anterior pituitary gland and the thyroid gland. The thyroid gland is located at the base of the neck and wraps around the trachea just below the cricoid cartilage .

Here is a zoomed in structure of the hypothalamus and pituitary gland. The hypothalamic–pituitary–thyroid axis (HPT axis) is under the control of neurons located in the medial region of the PVN that synthesize and release thyrotropin-releasing hormone (TRH) into the pituitary portal circulation. Thyrotropin-releasing hormone (TRH), which is a tripeptide, stimulates the release of thyrotropin (TSH) from the anterior pituitary, which in turn stimulates the synthesis and release of thyroid hormones.

Again thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis.

Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas if there is high TH the opposite occurs, there will be low T3 and T 4.

The functional unit of the thyroid gland are the follicles made up of follicular cells. The centre of the follicle is the colloid. The thyroid gland has a rich blood supply. Here are capillaries. Next to the follicles are the parafollicular cells also known as C cells which produce calcitonin another hormone, which does not really have an important role in humans.”

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