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Thyroid Physiology

Thyroid Regulation  

• Abraham

• Boeynaems, Braekman, Dumont

• Brix

• Carrasco, Eng

• Dugrillon, Gartner, Pisarev

• Fozzatti

• Rapoport

• Wolff

THYROID REGULATION

How is the thyroid regulated?  What determines how much iodide is in the thyroid?  What determines how many hormones it produces and secretes?

TRH-TSH-Thyroid Feedback Loop.

The simple -- and yet incomplete -- answer is that the hypothalamus, pituitary, and thyroid participate together in a carefully orchestrated feedback loop.  Low levels of thyroid hormones stimulate the release of TRH by the hypothalamus, which then stimulates the release of TSH by the pituitary.  When the TSH reaches the thyroid, it stimulates the production and release of thyroid hormones.

Adequate thyroid hormones in the blood inhibit the release of TRH by the hypothalamus, which then reduces the TSH released by the pituitary, which then reduces the amount of thyroid hormones produced and released by the thyroid.

In principle, this carefully orchestrated loop is always making sure that the thyroid hormone levels stay exactly right.  (See Diagram.)  Unfortunately, the actual situation is more complicated and not yet fully understood.

Raw Materials and Machinery.

For example, in order to make the hormones, the thyroid must have enough of the raw materials and the necessary machinery.  It must have enough iodine, tyrosine, functioning NIS, thyroid peroxidase, hydrogen peroxide, thyroglobulin, and all the other necessary components in good working order.

Autoregulation.

"Autoregulation" is the ability of the thyroid to regulate its own function and growth depending on the intrathyroidal availability of iodide.  For example, when the levels of iodine in the blood are "excessive", the formation of hormones can be inhibited.  This inhibition is called the "Wolff-Chaikoff effect", after the authors of articles that first described this phenomenon.

This inhibition does not last for very long.  It is normally transient, lasting from 26 to 50 hours.  This phenomenon is often referred to as "the escape from the Wolff-Chaikoff effect".

This inhibition and escape is not yet fully understood and does not always happen in the same way.  Thus, what is "excess" iodine for one person in one set of circumstances may not be "excess" for another person in a different set of circumstances.  Some thyroids may become "inhibited" and not "escape".  Sometimes, certain cells within the thyroid may act in one way, while other cells act in a different way.  Thus, it is not possible to give a universal answer to the question of how much iodine is "enough" and how much is "excessive".

These complex processes are still being unraveled and provide exciting opportunities for new discoveries.  But they create confusion for individuals trying to determine how much iodine is optimal.

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  Copyright: Zoe, 2006.