Endocrine System Foundations Hormone Axes

An endocrine axis is a chain of messages. The hypothalamus releases a signaling hormone, the anterior pituitary releases a trophic hormone, and a target gland produces the final hormone that acts on tissues throughout the body. The final hormone also feeds back to the hypothalamus and pituitary. This negative-feedback system prevents excessive hormone production. When the final hormone falls, pituitary stimulation should rise. When the final hormone rises, pituitary stimulation should fall. That direction of change helps locate the defect: - Primary gland failure: the target gland cannot produce enough hormone. The final hormone is low, while the pituitary trophic hormone rises in an attempt to stimulate the failing gland. - Central failure: the hypothalamus or pituitary does not provide enough stimulation. The final hormone is low, but the trophic hormone is low or inappropriately normal rather than elevated. - Autonomous hormone excess: a gland produces hormone without responding to pituitary control. The final hormone is high and the trophic hormone is suppressed. A “normal” pituitary value is not always reassuring. A normal TSH with a low free T4 is...