Endocrine System: Hormone Types and Regulation

The Endocrine System

Hormone Classification and Mechanisms

The endocrine system regulates physiological processes through the secretion of hormones. Hormones can be classified based on their solubility, which affects their transport, receptor location, and mechanism of action.

Comparison of Lipid-Soluble and Water-Soluble Hormones

Property	Lipid-Soluble Hormones	Water-Soluble Hormones
Chemical Nature	Steroids, thyroid hormones	Amino acid derivatives, peptides, proteins
Transport in Blood	Bound to plasma proteins	Dissolved in plasma (free form)
Receptor Location	Intracellular (cytoplasm or nucleus)	Cell membrane
Mechanism of Action	Direct gene activation	Second messenger systems (e.g., cAMP)
Onset and Duration	Slower onset, longer duration	Faster onset, shorter duration

Example: Cortisol (lipid-soluble) vs. insulin (water-soluble).

Pituitary Hormones: Regulation and Effects

The pituitary gland, often called the "master gland," secretes hormones that regulate many body functions. These hormones are controlled by hypothalamic signals and feedback mechanisms.

Hormone	Regulation	Main Effects
Growth Hormone (GH)	Stimulated by GHRH, inhibited by somatostatin	Stimulates growth, protein synthesis, fat breakdown
Thyroid-Stimulating Hormone (TSH)	Stimulated by TRH, inhibited by thyroid hormones	Stimulates thyroid hormone release
Adrenocorticotropic Hormone (ACTH)	Stimulated by CRH, inhibited by cortisol	Stimulates cortisol release from adrenal cortex
Luteinizing Hormone (LH)	Stimulated by GnRH, inhibited by sex hormones	Stimulates sex hormone production
Follicle-Stimulating Hormone (FSH)	Stimulated by GnRH, inhibited by sex hormones	Stimulates gamete production
Prolactin (PRL)	Inhibited by dopamine	Stimulates milk production
Antidiuretic Hormone (ADH)	Stimulated by increased plasma osmolality	Promotes water reabsorption in kidneys
Oxytocin	Stimulated by cervical/uterine stretching, suckling	Stimulates uterine contractions, milk ejection

Example: Increased TSH leads to increased thyroid hormone production, which in turn inhibits further TSH release (negative feedback).

Pancreatic Hormones

The pancreas has both endocrine and exocrine functions. Its endocrine portion (islets of Langerhans) secretes hormones crucial for glucose homeostasis.

• Insulin: Lowers blood glucose by promoting cellular uptake and storage of glucose as glycogen.

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

• Somatostatin: Inhibits secretion of both insulin and glucagon, regulating overall hormone balance.

Example: After a meal, insulin secretion increases to lower blood glucose; during fasting, glucagon secretion increases to raise blood glucose.