Hormone Classes: Steroid, Non-Steroid & Amino Acid

Chemical Classification of Hormones and Receptors

Steroid Hormones

• Steroid hormones are derived from cholesterol
• Steroid Hormones are produced by the adrenal glands and gonads.
• Steroid Hormones are hydrophobic (non-polar) and travel through blood bound to transport proteins
• Steroid Hormones diffuse across the phospholipid membrane of target cells.
• Steroid Hormones bind to intracellular receptors, activating genes in the nucleus.

Non-Steroid Hormones

• Non-Steroid Hormones includes peptides and proteins
• Non-Steroid Hormones are hydrophilic and do not require transport proteins.
• Non-Steroid Hormones bind to cell surface receptor proteins since they cannot pass through the cell membrane.
• Non-Steroid Hormones activate second messenger systems to alter gene expression.

Amino Acid Hormones

• Amino Acid Hormones are derived from tyrosine
• Examples of Amino Acid Hormones include:
  • Thyroid hormones (T3, T4)
  • Catecholamines (epinephrine and norepinephrine)

Pituitary Gland and Hypothalamus

Hypothalamus

• The hypothalamus regulates anterior pituitary activity.
• The hypothalamus monitors body temperature, metabolism, and electrolyte balance.
• The hypothalamus communicates via vascular and neural pathways.

Hypothalamic Hormones

• Hormones include:
  • Thyrotropin-releasing hormone (TRH)
  • Corticotropin-releasing hormone (CRH)
  • Gonadotropin-releasing hormone (GnRH)
  • Growth hormone-releasing hormone (GHRH)
  • Somatostatin (GHIH) inhibits growth hormone.
  • Dopamine (Prolactin-inhibiting hormone).

Pituitary Gland (Hypophysis)

• The anterior lobe (adenohypophysis) is derived from oral ectoderm
• The anterior lobe (adenohypophysis) produces hormones:
  • Growth hormone (GH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin (PRL).
• The posterior lobe (neurohypophysis) is derived from neural ectoderm
• The posterior lobe (neurohypophysis) stores and releases:
  • Antidiuretic hormone (ADH/vasopressin) which regulates water balance.
  • Oxytocin induces uterine contractions and lactation.

Thyroid and Parathyroid Glands

Thyroid Gland

• The thyroid gland has two lobes connected by the isthmus
• The location of the thryoid gland is anterior to the trachea and inferior to the larynx
• The thryoid gland produces:
  • Thyroxine (T4) and Triiodothyronine (T3) which regulate metabolism
  • Calcitonin which reduces blood calcium levels

Parathyroid Glands

• Four small parathyroid glands are located on the posterior thyroid
• Parathyroid glands secrete Parathyroid Hormone (PTH):
  • Parathyroid Hormone increases blood calcium levels by stimulating osteoclast activity

Pancreas

Anatomy

• The pancreas is a retroperitoneal organ
• The pancreas functions both exocrine (digestive enzymes) and endocrine
• The pancreas is divided into five parts: Head, Uncinate process, Neck, Body, and Tail.

Endocrine Function: Islets of Langerhans

• Beta cells secrete insulin which lowers blood sugar
• Alpha cells secrete glucagon which raises blood sugar
• Delta cells secrete somatostatin which inhibits GH and insulin
• PP cells secrete pancreatic polypeptide.

Adrenal Glands

Anatomy

• Adrenal glands are located on top of each kidney
• Adrenal glands secrete steroid hormones and catecholamines

Adrenal Cortex (Three Zones)

• Zona Glomerulosa cells secrete Aldosterone which regulates sodium balance
• Zona Fasciculata cells secrete Cortisol for stress response
• Zona Reticularis cells secrete Androgens

Adrenal Medulla

• Chromaffin cells secrete epinephrine and norepinephrine for sympathetic response

Blood Supply and Regulation of Endocrine Glands

• Thyroid gland is supplied by the superior and inferior thyroid arteries
• Parathyroid glands are supplied by the inferior thyroid arteries
• The pancreas is supplied by the splenic and pancreaticoduodenal arteries
• Adrenal glands are supplied by the suprarenal arteries and veins.

Hormonal Feedback Mechanisms

• Negative Feedback Loops:

  • T3 and T4 inhibit TRH and TSH production
  • Cortisol inhibits CRH and ACTH production
  • PTH regulates blood calcium through bone resorption

• Positive Feedback Loops:

  • Oxytocin during childbirth stimulates more contractions