Hormone feedback
How the hypothalamus, pituitary and target glands regulate one another through negative feedback, and how paired hormone levels show whether a disorder starts in the gland, the pituitary or outside the axis.
By the end, you can
- Describe a hypothalamic-pituitary-target gland axis and its negative feedback.
- Predict the hormone pattern of primary and secondary underactivity of a gland.
- Explain why autonomous hormone production suppresses the rest of its axis.
- Explain why stopping long-term steroid treatment suddenly can cause adrenal insufficiency.
- Explain the logic of stimulation and suppression tests.
- Recognise the classic hormone patterns, dynamic tests and classifications of axis disorders, and explain how replacement, suppression and removal of a hormone source act on feedback.
What it is
A hormone axis is a chain of glands in which each hormone stimulates the next: the hypothalamus releases a hormone that acts on the pituitary, whose hormone acts on a target gland. Negative feedback from the target gland’s hormone slows the chain, keeping its level steady. Endocrine disorders are described as primary (in the target gland), secondary (in the pituitary) or tertiary (in the hypothalamus).
3 types to tell apart
- Primary disorder
- The target gland fails or overacts; the pituitary hormone moves in the opposite direction to the gland’s hormone.
- Secondary or tertiary disorder
- The pituitary or hypothalamus fails or overacts; the pituitary hormone and the gland’s hormone move in the same direction.
- Autonomous secretion
- A tumour, a stimulating antibody or medication supplies the hormone regardless of feedback, and the rest of the axis is switched off.
Normal function
An axis with negative feedback
The integrates signals from the brain and body. Its neurones release hormones into small portal vessels in the , which carry them directly to the . The is different: it stores ADH and oxytocin made in the hypothalamus itself.
Each anterior pituitary hormone drives a target. Thyrotropin-releasing hormone (TRH) prompts the pituitary to release thyroid-stimulating hormone (TSH), which drives the thyroid to make thyroxine (T4) and triiodothyronine (T3). Corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) drive the adrenal cortex to make cortisol.
Thyroid hormone and cortisol feed back on the pituitary and hypothalamus and reduce their output. Like a thermostat, a small fall in the target hormone releases the brake and the axis speeds up; a rise does the reverse. A few loops use positive feedback instead, such as the surge of luteinising hormone before ovulation and oxytocin in labour.
The adrenal axis (CRH, ACTH, cortisol) and the gonadal axes work the same way.
Underlying change · Step 1 of 4
Primary disease: the gland itself
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Underlying change · Step 2 of 4
Secondary disease: the pituitary or hypothalamus
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 4
Autonomous secretion switches the axis off
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 4 of 4
Reading the pattern and testing the axis
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .