KEY POINTS

TheThyroid Gland

1. The thyroid hormones are synthesized from two connected tyrosine molecules that contain three or four iodine molecules.

2. Thyroid hormones are stored outside the cell and attached to thyroglobulin in the form of colloid.

3. The release of thyroid hormones involves transport of thyroglobulin with attached thyroid hormones into the cell, cleavage of the thyroid hormones from thyroxine-binding globulin, and release into the interstitial tissues.

4. Thyroid hormones are transported in the plasma attached to plasma proteins.

5. The main routes of metabolism of thyroid hormones are through deiodination or the formation of glucuronides and sulfates via hepatic mechanisms.

6. Thyroid hormones are the primary factors for the control of basal metabolism.

7. The ingestion of compounds that inhibit the uptake or organic binding of iodine blocks the thyroid's ability to secrete thyroid hormones and causes goiter.

The Adrenal Glands

1. The adrenal glands are composed of two organs: the outer gland (cortex) and the inner gland (medulla).

The Adrenal Cortex

1. The adrenal cortex has three zones: the zona glomerulosa, which secretes mineralocorticoids, and the zona fasciculata and the zona reticularis, which secrete glucocorticoids and sex steroids.

2. Adrenal corticoids are synthesized from cholesterol; the critical difference in the activity of these corticoids is related to the hydroxyl group on C-17 of glucocorticoids.

3. Adrenocortical hormones are carried in plasma in association with specific binding globulins (corticosteroid-binding globulin).

4. The metabolism of adrenocortical hormones involves the reduction of double bonds and conjugation of the steroids to glucuronides and sulfates.

5. One of the most important functions of glucocorticoids is control of metabolism, in particular the stimulation of hepatic gluconeogenesis.

6. Corticotropin is the pituitary hormone that regulates glucocorticoid synthesis by the adrenal cortex.

7. One of the most important clinical uses of glucocorticoids is the suppression of the inflammatory response.

The Adrenal Medulla

1. The synthesis of catecholamines is from tyrosine; the main catecholamine synthesized by the adrenal medulla is epinephrine.

2. The primary actions of catecholamines are on metabolism, especially effects that increase the concentration of glucose.

3. The main factors that stimulate catecholamine secretion are hypoglycemia and conditions that produce stress.

Hormones of the Pancreas

1. The synthesis of insulin is biphasic: an acute phase involves the release of preformed insulin, and a chronic phase involves the synthesis of protein.

2. The metabolism of insulin involves splitting the A and B chains and reducing the chains to amino acids and peptides.

3. The main metabolic functions of insulin are anabolic.

4. Insulin deficiency produces diabetes mellitus, which can culminate in diabetic ketoacidosis.

5. Dietary management is an important consideration in therapy for feline type 2 diabetes.

6. The most important functions of glucagon are to decrease glycogen synthesis, increase glycogenolysis, and increase gluconeogenesis.

7. Glucagon synthesis is stimulated by decreased glucose concentrations in the blood.

8. The main functions of somatostatin are to inhibit the secretion of hormones produced by the pancreas (insulin, glucagon, pancreatic polypeptide).

Calcium and Phosphate Metabolism

1. Calcium is important for many intracellular reactions, including muscle contraction, nerve cell activity, release of hormones through e?ocytosis, and activation of enzymes.

2. Phosphorus is important for the structure of bone and teeth, and organic phosphate serves as part of the cell membrane and several intracellular components.

3. The most important body pool of calcium involved in homeostasis is the extracellular fluid component.