The Endocrine Hierarchy
The endocrine system operates as a hierarchy with the hypothalamus at the apex, the anterior pituitary in the middle, and peripheral glands at the base.
Standalone Glands (Not Under Anterior Pituitary Control)
| Gland | Regulatory Stimulus | Hormone | Key Function |
|---|---|---|---|
| Parathyroid | Low blood Ca2+ | PTH | Raises blood Ca2+ |
| Thyroid C-cells | High blood Ca2+ | Calcitonin | Lowers blood Ca2+ |
| Adrenal medulla | Sympathetic nerve impulse | Adrenaline/NA | Fight-or-flight |
| Pancreatic β-cells | High blood glucose | Insulin | Lowers blood glucose |
| Pancreatic α-cells | Low blood glucose | Glucagon | Raises blood glucose |
| Pineal gland | Darkness (via SCN) | Melatonin | Circadian rhythm |
| Thymus | Bone marrow T-cell precursors | Thymosins | T-cell maturation |
The organs shown above respond directly to their physiological stimuli rather than through a hypothalamic-pituitary relay. This architectural distinction means that disorders in these glands are described as "primary" dysfunction — not secondary to pituitary failure.