The thyroid is an important organ that controls the metabolism of the body, which is the process by which your body converts what you eat and drink into energy. However, it can be difficult to understand the extent to which your day-to-day life can be affected if the thyroid is not functioning properly. We’ve designed this diagram of the Hypothalamus – Pituitary – Thyroid (or HPT) axis so it’s easy to understand, but we’ve also elaborated further below.
I find it helpful to imagine this pathway as a cascade of dominos. Each falling domino causes another domino to fall and so on. If all the dominoes are lined up well, then everything goes smoothly. However, any disruptions along the way can mean problems and cause thyroid-related issues. Here’s how it’s all supposed to work:
- The hypothalamus, located in the brain, senses the level of thyroxine (T4) and triiodothyronine (T3) in the body and then responds accordingly. The hypothalamus releases thyrotropin-releasing hormone (TRH) if the levels of T4 are low. Think of this as the first domino in the sequence.
- TRH stimulates the pituitary gland to produce another hormone called thyroid-stimulating hormone (TSH).
- TSH stimulates the thyroid gland to produce the hormones thyroxine (T4) as well as some triiodothyronine (T3). T4 is produced in larger amounts but is inactive.
- The inactive T4 is converted to active T3 in organs like the liver and kidneys. T4 and T3 hormones are taken up by tissues and cells to fuel the essential functions of the body.
- The new levels of T4 and T3 in the body are detected once more by that hypothalamus. If these levels are too high, then the hypothalamus produces less TRH. So, then less TRH means less TSH which means less T4 and T3… But then if the hypothalamus registers that the levels of T4 and T3 are too low, it increases TRH again, and the whole pathway continues in this self-regulating way.
That is how the HPT axis is supposed to work. There are a number of reasons why this may not go according to plan, which will be explored in another post!
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