Regulation and release of the thyroid hormones occurs as a negative feedback loop. What does this mean?

In simple terms, a negative feedback loop means that as something increases, the production of whatever is causing the increase slows down. So, picture a bowl of water that you want to keep full, but not let overflow. When it’s full, you stop adding water. If there’s a leak, you add water slowly to maintain the level. If it spills, you quickly add more. So when the volume of water increases, you decrease the addition of more water; if the bowl empties (the volume decreases), you increase the addition of more - hence the term, negative feedback.

The thyroid hormones, T3 and T4, are regulated in much same way. When levels of T3 and T4 decrease below normal, the hypothalamus releases thyroid regulating hormone (TRH), stimulating the pituitary gland to produce thyroid stimulating hormone (TSH), which acts on the thyroid gland to produce more hormones and raise the blood levels. Once the levels rise, the hypothalamus “shuts off” and stops secreting TRH, which in turn inhibits the pituitary gland release of TSH.