negative feedback
Negative feedback is like a household thermostat: when the room gets warm enough, the heater shuts off. In the body, when a hormone or its downstream product rises to a sufficient level, it signals back to the glands that produced it and tells them to slow down. This keeps the level from climbing endlessly and pulls it back toward a stable target.
Most endocrine axes run on this principle. In the thyroid axis, for example, thyroid hormone in the blood feeds back on the hypothalamus and pituitary, suppressing TRH and TSH so that thyroid hormone production eases off. The same logic governs cortisol acting back on CRH and ACTH, and sex steroids acting back on GnRH, LH, and FSH for much of the menstrual cycle.
Negative feedback is the dominant control mode of the endocrine system because it is stabilizing: deviations in either direction are automatically corrected. Clinically, it is why a failing gland causes the upstream stimulating hormone to rise (low thyroid hormone leads to high TSH), and why giving an outside hormone can suppress the body's own production.
In primary hypothyroidism the thyroid cannot make enough thyroid hormone, so negative feedback is lost and TSH rises markedly — an elevated TSH is the classic screening clue.
A failing target gland unmasks negative feedback by letting the upstream hormone climb.
Feedback can act at more than one level: long-loop feedback is the final hormone acting on hypothalamus and pituitary, while short-loop feedback is a pituitary hormone acting back on the hypothalamus.