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🫀 Science · Human Anatomy and Physiology · Lesson 6 of 12 · 5 min

Endocrine system: glands, hormones and feedback loops

⚖️ Hormones are the body's slow, long-distance messages. Match each gland to its hormone and you can reason through almost any feedback-loop question.

💡 Key idea

Negative feedback reverses a change to keep a set point; positive feedback amplifies a change until an event ends.

🧠 The idea

Endocrine glands secrete HORMONES into the blood; each acts only on target cells with matching receptors. The HYPOTHALAMUS controls the PITUITARY. The anterior pituitary releases growth hormone (GH), TSH, ACTH, FSH, LH and prolactin; the posterior pituitary releases ADH (water reabsorption in the kidneys) and oxytocin (uterine contractions, milk ejection), both made in the hypothalamus. The thyroid makes T3 and T4 (metabolic rate) and calcitonin (lowers blood calcium); the parathyroids make PTH (raises blood calcium). The adrenal cortex makes cortisol (stress, raises glucose) and aldosterone (retains sodium and water); the adrenal medulla makes epinephrine and norepinephrine. In the pancreas, beta cells release INSULIN (lowers blood glucose) and alpha cells release GLUCAGON (raises it). The pineal gland makes melatonin, the thymus makes thymosin for T cells, the ovaries make estrogen and progesterone, and the testes make testosterone. Most hormones are controlled by NEGATIVE FEEDBACK: the result shuts off the signal. Positive feedback amplifies the signal, as oxytocin does during labor.

✏️ Worked example

After a large meal, blood glucose rises. Beta cells in the pancreas sense it and release insulin, which helps cells take up glucose and the liver store it as glycogen. Glucose falls back to normal, and insulin release slows. That is NEGATIVE feedback. Now thyroid hormone: the hypothalamus releases TRH, the anterior pituitary releases TSH, and the thyroid releases T3 and T4. High T3 and T4 then inhibit TRH and TSH, so the loop turns itself down.

📌 Takeaways

  • Insulin lowers blood glucose; glucagon raises it.
  • PTH raises blood calcium; calcitonin lowers it.
  • Most hormone loops are negative feedback; oxytocin in labor is positive feedback.

📖 Terms in this lesson

Homeostasis: The body's process of keeping internal conditions such as temperature and blood sugar stable; for example, sweating to cool down on a hot day.

✅ Check yourself

Question 1 of 3

Which hormone increases water reabsorption by the kidneys?

  1. Thyroid-stimulating hormone
  2. Glucagon
  3. Calcitonin
  4. Antidiuretic hormone
Show the answer and rationale

Answer: D

ADH, released by the posterior pituitary, makes the kidney collecting ducts reabsorb more water. Calcitonin lowers blood calcium, glucagon raises blood glucose, and thyroid-stimulating hormone acts on the thyroid, not the kidneys.

Question 2 of 3

Blood calcium falls below normal. Which hormone release increases first?

  1. Calcitonin
  2. Insulin
  3. Antidiuretic hormone
  4. Parathyroid hormone
Show the answer and rationale

Answer: D

Low blood calcium triggers the parathyroid glands to release PTH, which raises calcium by stimulating osteoclasts and kidney reabsorption. Calcitonin does the opposite, insulin regulates glucose, and antidiuretic hormone regulates water, not calcium.

Question 3 of 3

Which situation is an example of positive feedback?

  1. Oxytocin release during labor
  2. Insulin release after a meal
  3. TSH falling as T4 rises
  4. Sweating as body temperature rises
Show the answer and rationale

Answer: A

Oxytocin strengthens contractions, which cause more oxytocin release, amplifying the change until birth. Insulin after a meal, sweating to cool down and TSH falling as T4 rises all reverse a change, which is negative feedback.

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