Endocrine System: Hypothalamus, Pituitary, and Associated Glands

Endocrine System Overview

The endocrine system is a network of glands that secrete hormones to regulate various physiological processes, including growth, metabolism, reproduction, and homeostasis. This study guide focuses on the hypothalamus, pituitary gland, and other major endocrine glands, summarizing their hormones, targets, and physiological effects.

Hypothalamus and Pituitary Gland

Hypothalamus

The hypothalamus is a region of the brain that links the nervous and endocrine systems. It produces releasing and inhibiting hormones that control the anterior pituitary gland and synthesizes hormones released by the posterior pituitary.

• GnRH (Gonadotropin-releasing hormone): Stimulates release of FSH and LH from anterior pituitary.

• GHRH (Growth hormone-releasing hormone): Stimulates release of GH.

• GHIH (Growth hormone-inhibiting hormone): Inhibits release of GH.

• TRH (Thyrotropin-releasing hormone): Stimulates release of TSH.

• CRH (Corticotropin-releasing hormone): Stimulates release of ACTH.

• PIH (Prolactin-inhibiting hormone): Inhibits release of prolactin.

Pituitary Gland

The pituitary gland is divided into anterior and posterior lobes, each releasing different hormones with specific targets and effects.

Posterior Pituitary

• Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding.

• ADH (Antidiuretic hormone): Not shown in the diagram, but important for water reabsorption in kidneys. Additional info: ADH is also called vasopressin.

Anterior Pituitary

• PRL (Prolactin): Stimulates milk production in breasts.

• GH (Growth hormone): Stimulates growth, cell reproduction, and regeneration. Acts on liver (to produce IGFs), skeletal muscle, and bone.

• TSH (Thyroid-stimulating hormone): Stimulates thyroid gland to release thyroid hormones (TH).

• ACTH (Adrenocorticotropic hormone): Stimulates adrenal cortex to release corticosteroids.

• FSH (Follicle-stimulating hormone): Stimulates sperm production in testes and egg production in ovaries.

• LH (Luteinizing hormone): Stimulates testosterone production in testes and estrogen/progesterone production in ovaries.

Summary Table: Hypothalamic-Pituitary Axis

Hormone	Source	Target	Main Effect
GnRH	Hypothalamus	Anterior Pituitary	Stimulates FSH & LH release
FSH	Anterior Pituitary	Testes/Ovaries	Sperm/Egg production
LH	Anterior Pituitary	Testes/Ovaries	Testosterone/Estrogen & Progesterone production
GH	Anterior Pituitary	Liver, Muscle, Bone	Growth, IGF production
TSH	Anterior Pituitary	Thyroid	Stimulates TH release
ACTH	Anterior Pituitary	Adrenal Cortex	Stimulates corticosteroid release
PRL	Anterior Pituitary	Breasts	Milk production
Oxytocin	Posterior Pituitary	Uterus, Breasts	Contractions, Milk ejection

Adrenal Glands

The adrenal glands sit atop the kidneys and consist of the adrenal cortex and adrenal medulla, each producing different hormones.

• Adrenal Cortex: Releases corticosteroids (e.g., cortisol) for long-term stress response.

• Adrenal Medulla: Releases catecholamines (e.g., norepinephrine, epinephrine) for short-term stress response.

Thyroid and Parathyroid Glands

Thyroid Gland

• Thyroid Hormones (TH): Increase metabolic rate, growth, and development.

• Calcitonin: Lowers blood calcium by increasing calcium storage in bones.

Parathyroid Gland

• PTH (Parathyroid hormone): Increases blood calcium by stimulating calcium release from bones, increasing calcium reabsorption in kidneys, and activating vitamin D.

Comparison Table: Thyroid vs. Parathyroid Hormones

Hormone	Source	Main Effect
Calcitonin	Thyroid	Increases calcium storage in bones (lowers blood calcium)
PTH	Parathyroid	Increases blood calcium (bone resorption, kidney reabsorption, vitamin D activation)

Pancreas

The pancreas has both endocrine and exocrine functions. Its endocrine portion regulates blood glucose via two main hormones:

• Insulin: Lowers blood glucose by increasing cellular uptake of glucose and promoting glycogenesis.

• Glucagon: Raises blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver.

Key Terms

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

Pineal Gland

The pineal gland is a small endocrine gland in the brain that produces melatonin, a hormone that regulates circadian rhythms (day/night cycles).

• Melatonin: Promotes sleep and helps regulate biological rhythms.

Summary Table: Major Endocrine Glands and Hormones

Gland	Hormone(s)	Main Target(s)	Main Effect(s)
Hypothalamus	Releasing/Inhibiting hormones	Pituitary gland	Regulate pituitary hormone release
Pituitary (Anterior)	GH, TSH, ACTH, FSH, LH, PRL	Various	Growth, metabolism, reproduction, lactation
Pituitary (Posterior)	Oxytocin, ADH	Uterus, kidneys, breasts	Contractions, water balance, milk ejection
Thyroid	TH, Calcitonin	Most cells, bones	Metabolism, calcium storage
Parathyroid	PTH	Bones, kidneys	Increase blood calcium
Adrenal Cortex	Corticosteroids	Many tissues	Long-term stress response
Adrenal Medulla	Norepinephrine, Epinephrine	Many tissues	Short-term stress response
Pancreas	Insulin, Glucagon	Liver, tissues	Regulate blood glucose
Pineal	Melatonin	Brain	Regulate circadian rhythms

Key Equations

• Gluconeogenesis: $2 \text{Pyruvate} + 4 \text{ATP} + 2 \text{GTP} + 2 \text{NADH} + 6 \text{H}_2\text{O} \rightarrow \text{Glucose} + 4 \text{ADP} + 2 \text{GDP} + 6 \text{P}_i + 2 \text{NAD}^+ + 2 \text{H}^+$

• Glycogenolysis: $\text{Glycogen} + n \text{Pi} \rightarrow n \text{Glucose-1-phosphate}$

Additional info: The above guide includes inferred details about ADH and the general functions of the adrenal medulla, as these are standard components of the endocrine system even if not explicitly shown in the provided diagrams.