Anatomy and Physiology / Unit 7: The Endocrine System
7.6 Hormone Regulation and Feedback
Notes
Most endocrine glands are controlled by negative feedback, often through the hypothalamic-pituitary axis.
- Thyroid example: low T3/T4 leads the hypothalamus to release TRH, so the anterior pituitary releases TSH, so the thyroid releases T3/T4. Rising T3/T4 then inhibits TRH and TSH.
- Calcium: low blood calcium leads to PTH from the parathyroids, which raises calcium (bone release, kidney reabsorption, activated vitamin D to absorb more from food). High calcium leads to calcitonin, which lowers it.
- Blood pressure and water: low blood volume leads to ADH and aldosterone, which retain water and sodium.
- Positive feedback examples: oxytocin during labor and milk ejection.
- Hormone levels can be too high or too low because of tumors, autoimmune disease or damage to a gland.
Practice
A patient with a tumor makes excess thyroid hormone. What happens to TSH levels due to negative feedback?
High T3/T4 inhibits the anterior pituitary, lowering TSH.
Blood calcium is below normal. Which sequence follows?
Parathyroid glands detect low calcium and release PTH.
Why is the thyroid hormone system an example of negative feedback?
The product inhibits its own stimulating hormones, returning levels to the set point.
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