Endocrine System and Hormone Overview

Overview of the Endocrine System

• The endocrine system uses hormones to regulate various bodily functions.
• The system consists of glands that secrete hormones and organs with partial endocrine functions.

Endocrine Glands

• Major endocrine glands include the pineal, pituitary, thyroid, parathyroid, pancreas (islets), adrenal, ovaries, and testes.
• Organs with partial endocrine functions are the hypothalamus, liver, thymus, heart, kidney, stomach, duodenum, skin, and placenta.
• The pineal gland is located in the brain.

The Brain Review

• Key structures in the brain included are the cerebrum, cerebellum, thalamus, hypothalamus, pituitary gland, pons, medulla oblongata, spinal cord and pineal body can be visualised in a median section of a sheep brain.

Hormone Chemistry

• Hormones are classified into amine, peptide, protein, and steroid hormones.
• Water-soluble hormones dissolve easily in the blood but require a receptor on the cell surface because don't cross lipid bilayer.
• Lipid-soluble hormones need a carrier protein to travel through the blood and can directly act on the cytoplasm because it does cross lipid bilayers.
• Cholesterol-derived hormones can pass through the lipid bilayer.

Intercellular Communication Methods

• Intercellular communication occurs through endocrine, paracrine, autocrine, synaptic, and neuroendocrine signaling.

Fate of Hormones

• Hormones that circulate freely last a short time and are broken down by the liver or kidneys.
• Hormones bound to a carrier protein last longer in the blood and serve as a reserve.

Feedback Systems

• The endocrine system uses negative and positive feedback systems.
• An example of negative feedback would be maintaining ideal body temperature
• Negative feedback loops are common in the endocrine system.
• An example of a positive feedback loop involves carbon dioxide in the atmosphere.

Hypothalamus

• The hypothalamus connects the conscious mind with the subconscious.
• The hypothalamus produces releasing hormones (RHs) and inhibiting factors.
• The hypothalamus communicates with the pituitary gland through a portal system of blood vessels.

Pituitary Gland

• The pituitary gland has two portions, the anterior (adenohypophysis) and posterior (neurohypophysis).
• The posterior pituitary only serves to store and regulate the release of hormones.

Hormones

• The hypothalamus sends releasing and inhibiting hormones to the anterior pituitary.

Posterior Pituitary Hormones

• The hypothalamus makes oxytocin, and antidiuretic hormone (ADH or vasopressin) which are then stored in the posterior pituitary gland.

Growth Hormone (GH)

• GH is released from the anterior pituitary and stimulated by GHRH.
• GH promotes bone and muscle growth and regulates metabolism.
• GH promotes anabolism of proteins and catabolism of lipids.

Diseases

• Acromegaly and pituitary dwarfism are diseases related to GH production.

Bovine Somatotropin (BST)

• BST is a growth hormone used in cows, which has been banned in Canada due to increased side effects.

Prolactin

• Prolactin begins and maintains lactation.
• Sensory input from mammary glands feeds back to the hypothalamus to release PRH.

Oxytocin

• Oxytocin is stored in the posterior pituitary
• Oxytocin promotes milk let-down in mammary glands and contraction of the uterus.

Thyroid-Stimulating Hormone (TSH)

• TSH is produced by the anterior pituitary.
• TSH acts on the thyroid gland, promoting its development and hormone manufacturing.
• The control loop involves the release of TRH from the hypothalamus
• The process has negative feedback loops.

Thyroid Gland

• The thyroid gland consists of microscopic follicles and is bilobed.
• Produces thyroid hormone and calcitonin.

Thyroid Hormone

• T4 = thyroxine
• T3 = triiodothyronine and is the active hormone.
• Thyroid hormones regulates the metabolic rate, helps control body temperature, and controls rate of calorie usage.

Thyroid Hormone Effect

• Production is lowered with stress
• Is key to CNS development
• Helps with anabolism of proteins
• Helps with catabolism of muscles
• Increases lipid and glucose

Thyroid Diseases

• Goiter: caused by iodine deficiency, leads to hypertrophy of thyroid follicles.
• Supplementing diet with lodized salt may assist.

Hyperthyroidism

• Hyperthyroidism is the overproduction of thyroid hormone.
• Hyperthyroidism is common in older cats and can be caused by benign or malignant tumors.
• Treatments include surgery, iodine- limited diets, thryoid medication and/or radioactive iodine.

Hypothyroidism

• Hypothyroidism is commonly seen in some dog breeds which may be autoimmune
• Hair loss, dry skin, lethargy, heat-seeking and weight gain are symptoms
• Can be controlled with hormone supplementation.

Calcitonin

• Calcitonin is made by C cells within the thyroid gland.
• Calcitonin works against homeostasis of blood calcium levels.
• It takes calcium from the bloodstream and puts it in bones and decreases calcium uptake by GIT.