Human Physiology I · Endocrine Physiology
Major Endocrine Axes and Calcium Homeostasis
On this page 7 sections
In 30 seconds
Three major axes integrate the hypothalamus, pituitary, and a peripheral gland: the HPT axis (thyroid hormone), the HPA axis (Cortisol Glucocorticoid raising glucose, suppressing inflammation Full entry →), and — with the pancreatic islets — blood-glucose control by Insulin Beta-cell hormone that lowers blood glucose Full entry → and Glucagon Alpha-cell hormone that raises blood glucose Full entry →. Each is governed by negative feedback that returns its regulated variable toward a set point. Calcium is regulated separately by parathyroid hormone, Calcitonin Lowers calcium by inhibiting bone resorption Full entry →, and vitamin D3 acting on bone, kidney, and intestine, with PTH as the dominant defender against hypocalcemia.
Why this matters
Axis testing relies on the feedback logic: measuring TSH with free T4 (HPT), ACTH with cortisol (HPA), or glucose with insulin reveals whether a defect is in the gland or in the controller. Calcium disorders are evaluated by measuring calcium together with PTH and vitamin D. Specific diagnostic criteria, lab reference ranges, and treatment protocols vary by institution and jurisdiction; these notes support education and do not replace clinical instruction or supervision. Urgent symptoms require immediate evaluation by qualified clinicians or local emergency services.
The college version
1. The HPT axis and thyroid hormone
In the Hypothalamic-pituitary-thyroid (HPT) axis TRH → TSH → thyroid hormone Full entry →, TRH from the hypothalamus stimulates TSH from the anterior pituitary, and TSH stimulates the thyroid to make thyroid hormone (T4 and T3), which raises metabolic rate, heat production, and protein synthesis. HPT feedback T3/T4 suppress TRH and TSH Full entry → is classic long-loop negative feedback: rising T4/T3 suppress TRH and TSH, and falling levels release that brake, keeping thyroid hormone near its set point.
2. The HPA axis and cortisol
In the Hypothalamic-pituitary-adrenal (HPA) axis CRH → ACTH → cortisol Full entry →, CRH → ACTH → cortisol from the adrenal cortex. Cortisol mobilizes fuels (raising blood glucose), suppresses inflammation, supports vascular tone, and helps the body cope with stress. HPA feedback Cortisol suppresses CRH and ACTH Full entry → is negative: cortisol inhibits CRH and ACTH. The HPA axis also shows a circadian rhythm and is activated by stress, which temporarily raises its set point.
3. Endocrine pancreas and glucose homeostasis
The Endocrine pancreas Islets secreting insulin, glucagon, somatostatin Full entry → consists of the islets of Langerhans, whose beta cells secrete insulin (lowers glucose by promoting uptake and storage), alpha cells secrete glucagon (raises glucose by promoting glycogenolysis and gluconeogenesis), and delta cells secrete somatostatin (which locally inhibits both insulin and glucagon, acting as a paracrine brake). Blood-glucose homeostasis Balance keeping glucose in a narrow range Full entry → is the balance of these opposing hormones, holding plasma glucose in a narrow range across fed and fasted states.
4. Calcium homeostasis
Calcium homeostasis Stable extracellular calcium Full entry → keeps extracellular calcium stable for nerve, muscle, and blood-clotting function. Parathyroid hormone (PTH) Raises calcium via bone, kidney, intestine Full entry → is released when calcium falls; it raises calcium by acting on bone (stimulating osteoclast-mediated resorption), kidney (increasing calcium reabsorption and activating vitamin D), and intestine (indirectly, via vitamin D–driven absorption). Calcitonin, from thyroid C cells, lowers calcium by inhibiting bone resorption — a minor counterweight in adults. Vitamin D3 (calcitriol) is activated by PTH in the kidney and increases intestinal calcium and phosphate absorption.
How it works
- Each axis senses its regulated variable via the target gland, pancreas, or parathyroid.
- A low variable triggers secretion of the appropriate hormone.
- The hormone acts on effector tissues to move the variable back toward normal.
- Restoration of the variable removes the stimulus (negative feedback).
- Opposing hormones (insulin vs glucagon; PTH vs calcitonin) allow bidirectional control.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| HPT axis | HPA axis | Thyroid (metabolism/heat) vs adrenal cortex (cortisol/stress) |
| Insulin | Glucagon | Lowers vs raises blood glucose |
| T3/T4 feedback | Cortisol feedback | Both long-loop, but target different glands |
| PTH | Calcitonin | Raises vs lowers blood calcium |
| Vitamin D3 | PTH | D3 boosts gut absorption (PTH-activated); PTH resorbs bone/reabsorbs kidney calcium |
| Pancreatic somatostatin | Hypothalamic somatostatin | Both inhibitory, but pancreatic form acts locally on islets |
Memory aids
"TRH Tells TSH to Think (thyroid); CRH Calls ACTH for Cortisol." For glucose, "Insulin In; Glucagon Goes up." For calcium, "PTH Pulls calcium uP; Calcitonin Calms it down."
Quick review
Topic Recap
The HPT axis (TRH→TSH→thyroid hormone), the HPA axis (CRH→ACTH→cortisol), and the endocrine pancreas (insulin/glucagon/somatostatin) each hold a critical variable near a set point through negative feedback. Calcium homeostasis is governed chiefly by PTH (with calcitonin as a weak counterweight and vitamin D3 enabling gut absorption) acting on bone, kidney, and intestine. Together these loops illustrate negative-feedback integration across the endocrine system.
Knowledge Check
- Trace the HPT axis from hypothalamus to target hormone, and state the feedback signal.
- How does the HPA axis respond to stress, and how is it normally restrained?
- Which two pancreatic hormones oppose each other to keep blood glucose stable?
- List the three tissues PTH acts on and its effect at each.
- Why is calcitonin considered a minor regulator of calcium in adult humans?
Answers and Rationales
- Hypothalamic TRH → pituitary TSH → thyroid T3/T4; rising T3/T4 feed back to suppress TRH and TSH (long-loop negative feedback).
- Stress increases CRH → ACTH → cortisol, raising glucose and suppressing inflammation; normally cortisol feeds back to inhibit CRH and ACTH, but stress raises the set point.
- Insulin (lowers glucose) and glucagon (raises glucose); somatostatin from delta cells dampens both.
- Bone (release calcium via osteoclasts), kidney (reabsorb calcium, excrete phosphate, and activate vitamin D), and intestine (indirectly via vitamin D–stimulated absorption).
- Its effect on bone is weak and transient in adults, and PTH is the dominant homeostatic regulator; calcitonin plays a larger role in some other contexts and life stages.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of each axis as a thermostat. The hypothalamus-pituitary is the thermostat setting a target, the target gland is the furnace, and the hormone it makes is the heat. When the "heat" (hormone) gets high enough, the thermostat shuts the furnace off — negative feedback. For blood glucose, the pancreas is a thermostat with two switches: insulin lowers glucose (stores it away) and glucagon raises it (releases it), so the body always has fuel but never too much. Calcium has its own thermostat (PTH) that pulls calcium out of bone, kidney, and gut when blood levels drop too low.
The "where it stops being exact" part: these are not three independent thermostats — stress, feeding, and sleep all shift the set points, and the loops interact (cortisol raises blood glucose, thyroid hormone raises metabolic rate). Also, calcitonin is a weak player in adult humans; PTH does nearly all the work of defending calcium.
Simple Example
You skip breakfast, so blood glucose falls. Pancreatic alpha cells release glucagon, which tells the liver to break down glycogen and release glucose — glucose rises back to normal. After a meal, glucose spikes, beta cells release insulin, and insulin moves glucose into cells and into storage — glucose falls back to normal. Two opposing hormones hold one variable in a narrow range.
Worked example
- Blood calcium falls below its set point.
- Parathyroid chief cells sense the drop and secrete PTH.
- PTH acts on bone to release calcium and phosphate, and on the kidney to reabsorb calcium, excrete phosphate, and activate vitamin D3 (calcitriol).
- Calcitriol boosts intestinal calcium absorption; PTH plus calcitriol together raise plasma calcium.
- As calcium returns to normal, the stimulus for PTH fades — negative-feedback integration shuts PTH off.
- If calcium overshoots, calcitonin (weakly in adults) inhibits bone resorption.
- The HPT, HPA, and pancreatic axes hold their own variables near set points by the same feedback logic.
Key takeaways
- High yield: HPT axis: TRH → TSH → T3/T4, with T3/T4 providing long-loop negative feedback.
- High yield: HPA axis: CRH → ACTH → cortisol, with cortisol suppressing CRH/ACTH; stress activates and resets this loop.
- High yield: Insulin lowers blood glucose (uptake and storage); glucagon raises it (glycogenolysis and gluconeogenesis); both are needed for glucose homeostasis.
- PTH is the dominant calcium regulator, raising it via bone resorption, kidney reabsorption, and vitamin D–mediated gut absorption.
- Calcitonin lowers calcium by inhibiting osteoclasts but is a minor player in adult humans.
- Vitamin D3 must be activated by PTH in the kidney before it can boost intestinal calcium absorption.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Trace the hypothalamic-pituitary-thyroid (HPT) axis and explain how thyroid hormone exerts negative feedback.
- Trace the hypothalamic-pituitary-adrenal (HPA) axis and describe cortisol's metabolic and feedback roles.
- Explain how the endocrine pancreas uses insulin, glucagon, and somatostatin to maintain blood-glucose homeostasis.
- Describe calcium homeostasis through the coordinated actions of parathyroid hormone, calcitonin, and vitamin D3 on bone, kidney, and intestine.
Key vocabulary
- Hypothalamic-pituitary-thyroid (HPT) axis
- TRH → TSH → thyroid hormone
- Thyroid hormone (T3/T4)
- Hormone that raises basal metabolic rate
- HPT feedback
- T3/T4 suppress TRH and TSH
- Hypothalamic-pituitary-adrenal (HPA) axis
- CRH → ACTH → cortisol
- Cortisol
- Glucocorticoid raising glucose, suppressing inflammation
- HPA feedback
- Cortisol suppresses CRH and ACTH
- Endocrine pancreas
- Islets secreting insulin, glucagon, somatostatin
- Insulin
- Beta-cell hormone that lowers blood glucose
- Glucagon
- Alpha-cell hormone that raises blood glucose
- Somatostatin (pancreatic)
- Delta-cell hormone inhibiting insulin/glucagon
- Blood-glucose homeostasis
- Balance keeping glucose in a narrow range
- Calcium homeostasis
- Stable extracellular calcium
- Parathyroid hormone (PTH)
- Raises calcium via bone, kidney, intestine
- Calcitonin
- Lowers calcium by inhibiting bone resorption
- Vitamin D3 (calcitriol)
- Activated by PTH; boosts gut calcium absorption
- Bone/kidney/intestine
- The three calcium-handling tissues
- Negative-feedback integration
- Each axis self-limits via feedback
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