The Endocrine System: Structure, Function, and Hormone Classification

The Endocrine System

Overview of the Endocrine System

The endocrine system is the body's second major control system, working alongside the nervous system to regulate physiological processes. It influences the metabolic activities of cells through the release of hormones.

• Endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands.

• The pancreas and gonads (ovaries and testes) produce both hormones and exocrine products.

• The hypothalamus has both neural and endocrine functions, releasing hormones that regulate other endocrine glands.

• Other tissues and organs, such as adipose tissue, the gastrointestinal tract, kidneys, and heart, also produce hormones.

Major Endocrine Organs

The major endocrine organs are distributed throughout the body and are responsible for the synthesis and secretion of hormones that regulate various physiological functions.

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Parathyroid glands

• Thymus gland

• Adrenal glands

• Pancreas

• Ovary (female)

• Testis (male)

Autocrines and Paracrines

Local Chemical Messengers

Not all chemical messengers are considered hormones. Autocrines and paracrines are local chemical messengers with distinct functions:

• Autocrines: Chemicals that exert their effects on the same cells that secrete them.

• Paracrines: Locally acting chemicals that affect cells other than those that secrete them.

• These are not considered hormones because hormones are long-distance chemical signals.

Hormones: Definition and General Properties

What Are Hormones?

Hormones are chemical substances secreted by cells into the extracellular fluids. They regulate the metabolic function of other cells, have lag times ranging from seconds to hours, and tend to have prolonged effects.

• Hormones are classified as amino acid-based hormones or steroids.

• Eicosanoids are biologically active lipids with local hormone-like activity.

Classification of Hormones

Types of Hormones

Hormones can be classified based on their chemical structure, which determines their mechanism of action and physiological effects.

• Amino acid-based hormones (Amines):

  • Derived from amino acids such as tryptophan, histidine, and tyrosine.

  • Examples: Melatonin, serotonin, histamine, catecholamines (dopamine, norepinephrine, epinephrine), and thyroid hormones.

• Peptide/Polypeptide hormones:

  • Short chains of amino acids (peptides) or longer chains (polypeptides).

  • Examples: Thyrotropin-releasing hormone (3 amino acids), gonadotropin-releasing hormone (10 amino acids), glucagon (29 amino acids), insulin (51 amino acids).

• Protein hormones:

  • Larger, more complex molecules.

  • Examples: Luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH).

• Steroid hormones:

  • All derived from cholesterol.

  • Examples: Testosterone, estrogen, progesterone, cortisol, aldosterone.

• Fatty acid-derived hormones (Eicosanoids):

  • Derived from arachidonic acid.

  • Examples: Prostaglandins, leukotrienes, thromboxanes.

Summary Table: Hormone Classification

Class	Origin/Structure	Examples
Amino acid-based (Amines)	Derived from single amino acids	Melatonin, Serotonin, Catecholamines, Thyroid hormones
Peptide/Polypeptide	Short/long chains of amino acids	TRH, GnRH, Glucagon, Insulin
Protein	Large, complex proteins	LH, FSH, TSH
Steroid	Derived from cholesterol	Testosterone, Estrogen, Progesterone, Cortisol
Fatty acid-derived (Eicosanoids)	Derived from arachidonic acid	Prostaglandins, Leukotrienes, Thromboxanes

Key Points

• Most hormones are amino acid-based.

• Steroid hormones are synthesized from cholesterol and include gonadal and adrenocortical hormones.

• Eicosanoids act locally and are not considered true hormones by some definitions.

Example: Hormone Action

• Insulin (a peptide hormone) regulates blood glucose levels by promoting cellular uptake of glucose.

• Testosterone (a steroid hormone) influences the development of male secondary sexual characteristics.