Library · Physiology
TSH, T4, and T3: the thyroid feedback loop
Make it clearer
The thyroid makes mostly T4, along with some T3, two hormones that set the pace at which the body uses its energy and that nearly every organ depends on to work properly. TSH is a different thing, and it's worth not mixing them up: it isn't made in the thyroid. It comes from the pituitary, a gland that sits underneath the brain, and what it does is tell the thyroid how much hormone to produce.
The arrangement works like a thermostat. If little thyroid hormone is circulating in the blood, the pituitary tends to raise TSH; if too much is circulating, it tends to lower it. That back-and-forth, the feedback loop, keeps the signal adjusted to how much hormone is available at that moment. And that's exactly the limit: a TSH value on its own tells you something has shifted, but not where the shift is coming from.
In brief
The thyroid makes mostly T4 and some T3, hormones that influence energy use and the way nearly every organ works.
TSH is not made by the thyroid. It comes from the pituitary, a small gland just below the brain, and it tells the thyroid to make more or less hormone.
When less thyroid hormone is circulating, the pituitary usually raises TSH; when more is circulating, it lowers it. This feedback matches the signal to the hormone available, but TSH alone does not reveal the cause of a change.
Sources (2)
- National Institute of Diabetes and Digestive and Kidney DiseasesThyroid Tests
- National Library of MedicineTSH (Thyroid-stimulating hormone) test
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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