How the Endocrine Glands and Hormones Work

Hormones: Classification and Mechanism of Action

Lipid-soluble: steroids: modification of the cholesterol molecule.
Thyroid: iodination of two molecules linked to tyrosine (T3, T4).

Water-soluble: Amines: decarboxylation of amino acids.
Protein:

Action

Lipid-soluble: the hormone enters the cell, binds to receptors in the cytosol and nucleus, and modifies DNA, which is transcribed into mRNA and translated into active cell proteins.

Water-soluble: the hormone joins a membrane receptor (first messenger), which activates a G protein that in turn activates adenylate cyclase (ATP to cAMP; second messenger). cAMP activates protein kinases, which phosphorylate enzymes that carry out functions.

Hypothalamus

The hypothalamus is an endocrine gland that also receives input from other endocrine glands and secretes mediators that regulate endocrine organs.

Functions: it regulates the autonomic nervous system, body temperature, thirst, hunger, sexual impulses, and acts on growth processes.

Adenohypophysis (Anterior Pituitary, 85%)

Glandular epithelial tissue derived from the entoblast. Cells producing epithelial secretions are acidophilic, basophilic, and chromophobic, and secrete STH, TSH, ACTH, FSH, LH, ICSH, MSH, and PRL.

Posterior Pituitary Gland (15%)

Ectoderm-derived glial cells (nervous tissue). It stores and releases ADH and oxytocin, which are produced by the hypothalamus.

Somatotropin (STH) or Growth Hormone (GH)

Stimulates tissue growth by increasing the number of mitoses, increases body protein synthesis, and is lipolytic. It acts on growth processes, and its growth-promoting effects are stimulated by STH.

ADH (Antidiuretic Hormone)

Acts on the kidney, promoting water retention in the collecting tubules, and decreases sweating and causes vasoconstriction.

Oxytocin

Increases uterine contractions.

Thyroid Gland

Thyroid hormones: they are formed when iodine is oxidized and coupled to tyrosine. Iodine is captured from the diet. Thyroglobulin is formed in the rough endoplasmic reticulum. The iodide is oxidized and coupled, forming T1 and T2, which when active become T3 and T4.

Functions: increase the metabolic activity of the body, increase ATP production in tissues, increase cholesterol breakdown, favor temperature regulation and similar TG (triglyceride) effects, and interact with growth hormone (GH) and catecholamines.

Calcitonin

Controls blood calcium levels. It is produced by C cells of the thyroid and is hypocalcemic.

Parathyroid Gland

Parathyroid hormone: a protein with effects opposite to calcitonin (it increases calcium levels).

Pancreas

The pancreas is a mixed gland: its exocrine part secretes digestive enzymes, and its endocrine part is formed by the pancreatic islets (islets of Langerhans).

• Alpha cells: glucagon
• Beta cells: insulin
• Delta cells: somatostatin
• F cells: pancreatic polypeptide

Insulin and glucagon control blood glucose.

• Insulin: increases the permeability of the cell membrane, accelerates glycogen synthesis, stimulates lipid synthesis and protein synthesis, and potentiates the action of GH.
• Glucagon: hyperglycaemic; raises blood glucose levels.