Tropic Hormones: Master Regulators of the Endocrine Axis

Definition & Hierarchy

Tropic (trophic) hormones stimulate target endocrine glands to secrete their own hormones. They form the hypothalamic–pituitary–target gland axes.

Key Tropic Hormones

Tropic Hormone	Source	Target Gland	Target Hormone(s)
TRH	Hypothalamus	Anterior pituitary	TSH, Prolactin
CRH	Hypothalamus	Anterior pituitary	ACTH
GnRH	Hypothalamus	Anterior pituitary	LH, FSH
GHRH / Somatostatin	Hypothalamus	Anterior pituitary	GH (stimulate / inhibit)
TSH	Ant. pituitary	Thyroid	T₄, T₃
ACTH	Ant. pituitary	Adrenal cortex	Cortisol, androgens, aldosterone
LH / FSH	Ant. pituitary	Gonads	Estradiol, progesterone, testosterone

Regulatory Principles

• Negative feedback: Target hormone inhibits tropic hormone (e.g., cortisol ↓ CRH & ACTH).
• Ultradian/circadian rhythms: Pulsatile secretion (e.g., GnRH hourly, cortisol diurnal).
• Feed-forward: Estrogen positive feedback → LH surge (ovulation).

Clinical Disorders

• Primary gland failure → ↑ tropic hormone (e.g., primary hypothyroidism: ↑ TSH).
• Pituitary/hypothalamic disease → ↓ tropic hormone → secondary gland failure (e.g., Sheehan's: ↓ ACTH → adrenal insufficiency).
• Ectopic production (e.g., small cell lung cancer → ACTH → Cushing's).

Tropic hormones orchestrate endocrine homeostasis; their measurement distinguishes primary vs. secondary endocrine disorders.