Lecture 9: Study Guie

Endocrine System Overview

• Composed of groups of tissues → organs

• Ductless vascular tissues release hormones into circulation

• Hormone: "Signaling molecule"

  • Types: Protein, Steroid, Modified Amino Acids

• Hormones travel to distant targets, interact with receptors, and create effects

• Control biological processes and affect most tissues

• Endocrine dysfunction is common

Types of Hormones

Protein and Polypeptide Hormones

• Produced in endocrine tissues via gene transcription

• Released in response to specific signals (e.g., insulin secretion with increased glucose levels)

• Stored in large quantities for quick release

• Short half-lives

• Bind to cell surface receptors

• Diseases occur with receptor mutations (e.g., inactivation or nonhormonal pathway activation)

Steroid Hormones

• Derived from cholesterol

• Produced in:

  • Adrenal cortex

  • Ovaries/Testes

• Not stored in large quantities; synthesized rapidly from cholesterol

• Hydrophobic, pass through cell membranes easily

• Transported by carrier proteins in blood

• Longer half-lives, slow onset but longer-lasting effects

Modified Amino Acid Hormones

• Derived from tyrosine

• Examples:

  • Thyroid hormones

  • Epinephrine, Norepinephrine

• Mostly water-insoluble, transported via carrier proteins

• Act via intracellular receptors

• Short half-lives (<5 min)

Endocrine Disorders

Hypertrichosis (Cushing’s Syndrome) in Horses

• Excessive hair growth, failure to shed coat

• Common in older horses (>18 years)

• Pituitary Pars Intermedia Dysfunction (PPID)

  • Adenoma of pars intermedia

• Compared to hirsutism: Increased androgen levels lead to hair growth

• Affects hypothalamus: Alters body temp, appetite, shedding

Clinical Signs:

• PU/PD, muscle weakness, somnolence

• Abnormal fat distribution, periorbital swelling

• Increased infection risk, intermittent fever

• Pathognomonic sign: Long, curly hair coat

• Insulin resistance

Diagnosis:

• ACTH concentrations

• Blood cortisol response to steroids

• Thyrotropin-releasing hormone test

Treatment:

• Pergolide (dopaminergic receptor agonist)

• Frequent body hair clipping, supportive care

Hyperthyroidism in Horses

• Over-diagnosed

• Primary: Thyroid gland dysfunction

• Secondary: Anterior pituitary dysfunction → low TSH

Clinical Signs:

• Foals: Weakness, poor suckling, fine hair coat, tendon laxity, low temp

• Adults: Exercise intolerance, lethargy, low heart rate, obesity, laminitis

Diagnosis:

• Blood thyroid hormone levels (unreliable)

• TRH response test (rarely used)

• Response to thyroid hormone supplementation

Treatment:

• Thyroid hormone supplementation (weaning required)

Hypothyroidism in Sheep/Goats (Goiter)

• Caused by: Iodine deficiency, thyrotoxic plants

• Enlarged thyroid gland due to low T3/T4 → Increased TSH

Clinical Signs:

• Poor wool quality, dry skin

• Tendon laxity, poor reproductive performance

Diagnosis:

• Clinical signs + Low iodine levels

Treatment:

• Iodine supplementation

Hyperparathyroidism

• Caused by: Nutritional imbalance (Ca:P ratio), oxalate-rich plants

• Secondary to: Owner-induced (iatrogenic) conditions

• Parathyroid hormone (PTH) release causes bone calcium resorption

Clinical Signs:

• Intermittent lameness, inability to rise

• Loose teeth, spontaneous fractures

Diagnosis:

• Radiographs, dietary analysis

Treatment:

• Correct mineral imbalance

• Prevention with proper diet