Introduction to Endocrine Physiology and Hormones

Comparing Communication Systems

The endocrine system works in tandem with the nervous system to regulate body functions, but they utilize very different "languages" and timelines.

Classification of Hormones

Hormones are categorized into three main groups based on their chemical structure: peptides, steroids, and amines. Their chemical nature dictates how they travel in the blood, where their receptors are located, and how quickly they act.

Peptide Hormones

Peptide hormones consist of linked amino acids (e.g., insulin). They are lipophobic (water-soluble), meaning they dissolve easily in plasma but cannot cross the lipid bilayer of cell membranes. Consequently, they must bind to cell-surface receptors. This binding triggers "second messengers" that change the activity of existing proteins, resulting in a fast onset of action.

Steroid Hormones

Steroid hormones are derived from cholesterol (e.g., cortisol, testosterone). They are lipophilic (lipid-soluble), allowing them to diffuse directly through the plasma membrane. Their receptors are located inside the cell (in the cytoplasm or nucleus). Instead of changing existing proteins, they typically activate genes to synthesize new proteins, leading to a slow onset but a long duration of action.

Amine Hormones

Amine hormones are modified from single amino acids. This group is diverse: some (like catecholamines) behave like peptide hormones, while others (like thyroid hormones) behave like steroid hormones.

Hormone-Target Relationships: Examples

To understand how these chemicals function in a living organism, we can look at specific hormone-target pairings:

• Insulin: Secreted by the pancreas, it targets skeletal muscle to regulate glucose metabolism.
• Anti-diuretic hormone (ADH): Secreted by the posterior pituitary, it targets the kidneys to preserve water.
• Erythropoietin (EPO): Secreted by the kidneys, it targets bone marrow to stimulate red blood cell production.
• Prolactin: Secreted by the anterior pituitary, it targets the breasts for milk production.

Clinical Applications: The Cortisol and Thyroid Pathways

Many endocrine functions are controlled by complex "axes" involving the hypothalamus, the pituitary gland, and a target endocrine gland. These pathways are regulated by negative feedback to maintain homeostasis.

The Cortisol Pathway (Steroid)

Cortisol is a steroid hormone released from the adrenal cortex. It helps prevent low blood glucose and suppresses the immune system.

• Hypercortisolism (Cushing’s Syndrome): Too much cortisol can lead to hyperglycemia, central obesity, and a "moon face".
• Hypocortisolism: Too little cortisol can result from autoimmune destruction of the adrenal cortex.

The Thyroid Pathway (Amine)

Thyroid hormones (T3 and T4) are amine hormones that require iodine for synthesis. They regulate metabolism and thermogenesis in adults.

• Hyperthyroidism: Characterized by weight loss, rapid heart rate, and heat intolerance. A common cause is Grave’s disease, where antibodies mimic TSH and overstimulate the gland.
• Hypothyroidism: Characterized by fatigue, weight gain, and feeling cold. This can be caused by iodine deficiency or Hashimoto’s disease.

Key Concept: Feedback Loops

• Long-loop negative feedback: When the final hormone (e.g., Cortisol or T3/T4) inhibits the hypothalamus or pituitary.
• Short-loop negative feedback: When the pituitary hormone (e.g., ACTH) inhibits the hypothalamus.