Endocrine System Notes

Hormones and Their Importance

• Hormones are crucial for various bodily functions.
• The lecture will cover major endocrine organs, hormone classes, their effects, and secretion control.
• Endocrine system diseases in dogs, like thyroid hormone diseases, diabetes mellitus, and adrenal gland diseases, will be discussed.

Endocrine System vs. Nervous System

• Both systems are major communication systems in the body.
• They maintain homeostasis and integrate internal and external stimuli.
• Both consist of specialized cells and tissues but signal differently.

Nervous System

• Fast, point-to-point control.
• Anatomically connected, like telephone lines or a tree.
• Electrical impulses travel to specific targets, causing localized effects.
• Example: Raising an arm quickly due to a nervous impulse.

Endocrine System

• Anatomically diffused with endocrine organs throughout the body.
• Tissues make hormones that travel through the bloodstream to targets.
• Hormones target cells with specific receptors.

Principal Functions of the Endocrine System

• Maintains homeostasis (balanced internal environment).
• Regulates growth, metabolism, and development.
• Controls reproductive tract development, sexual reproduction, gametogenesis, fertilization, implantation, parturition, and newborn nourishment.

Overview of Endocrine Organs

• Thyroid gland (neck).
• Pituitary gland (base of the brain).
• Parathyroid glands (regulate calcium metabolism).
• Reproductive organs (ovaries in females, testes in males).
• Pancreas (makes insulin).
• Adrenal glands (make various hormones).

Structure of Hormones

• Three major classes: steroid hormones, protein hormones, and amino acid analogs/derivatives.
• Structure corresponds to signaling and effects.

Steroid Hormones

• Derived from cholesterol.
• Cholesterol is modified into different steroid hormones with unique actions.
• Tissues producing steroid hormones:
  • Ovaries (estrogen and progesterone).
  • Testes (androgens like testosterone).
  • Adrenal cortex (glucocorticoids and mineralocorticoids).
• Examples: testosterone, estradiol, mineralocorticoids, and glucocorticoids.
• Different tissues have different enzymes that modify cholesterol into specific steroid hormones.

Protein/Peptide Hormones

• Proteins or peptides produced by endocrine organs.
• Wide range of sizes, from small peptides to large proteins.
• Made by:
  • Anterior pituitary.
  • Thyroid.
  • Parathyroid.
  • Endocrine pancreas.
• Examples: insulin, prolactin, growth hormone.
• Posterior pituitary hormones:
  • Antidiuretic hormone (ADH) involved in water balance (9 amino acids).
  • Oxytocin involved in smooth muscle contraction and milk expression (9 amino acids).

Amino Acid Analogs/Derivatives

• Modified amino acids with hormonal properties.
• Made by:
  • Thyroid gland (thyroid hormone).
  • Adrenal medulla (epinephrine and norepinephrine).
• Tyrosine is a precursor for thyroid hormones (T4 and T3) and catecholamines (dopamine, norepinephrine, epinephrine).
• T4 (thyroxine) has four iodine molecules, T3 (triiodothyronine) has three.
• Catecholamines are derived from tyrosine including dopamine, norepinephrine, and epinephrine.

How Endocrine Organs Signal

• Hormones are released into the bloodstream and circulate throughout the body.
• Cells respond based on the presence of receptors.
• Hormones act on cells with receptors, leading to biological responses.
• Receptor presence determines if a cell responds to a hormone; absence means no response.

Modes of Action

Protein Hormones

• Bind to receptors on the cell surface.
• Binding triggers events inside the cell.
• Increases in second messengers (e.g., cyclic AMP, cyclic GMP, calcium) elicit biological effects.
• The hormone doesn't need to enter the cell.

Steroid Hormones

• Pass through the cell membrane.
• Bind to receptors inside the cell (cytoplasm or nucleus).
• Hormone-receptor complex translocates to the nucleus.
• Binds to DNA, inducing the production of new proteins.
• New proteins induce the final effects of the steroid hormone.
• Thyroid hormones act like steroid hormones with intracellular receptors.
• Catecholamines (epinephrine, norepinephrine, dopamine) act through surface receptors.

Control of Hormone Production

• Hormone production and secretion are tightly regulated.
• Mechanisms involved:
  • External stimuli (fright, cold, light cues).
  • Internal stimuli (blood sugar levels, signals from other hormones).

Feedback Loops

• The body is sensitive to hormone levels and regulates physiology with feedback loops.
• Examples of hormone balance:

Thyroid Gland and Thyroid Hormones

• Regulated by a feedback loop involving TRH (hypothalamus), TSH (pituitary), and T4/T3 (thyroid).
• The hypothalamus produces TRH, which stimulates the pituitary to produce TSH.
• TSH stimulates the thyroid to produce T4 and T3.
• T4 and T3 inhibit TRH and TSH production.
• Functions of thyroid hormones:
  • Increase metabolic rate.
  • Essential for growth in young animals.
  • Necessary for growth hormone secretion.
  • Critical for central nervous system development and maturation.

Thyroid Problems in Dogs

• Common in dogs, humans, and cats.

• Low thyroid hormone (Hypothyroidism) : metabolism is reduced. Underactive thyroid gland.

  • Cretinism: Thyroid hormone deficiency at birth which affects CNS and may not be reversed.

• High thyroid hormone (Hyperthyroidism): increased metabolic rate where Thyroid glands enlarge.

  • Enlargement of the thyroid gland.
  • Animal can become irritable and nervous
  • Exophthalmia: bulging eyeballs of animals that are hyperthyroid

Thyroid Disease in Dogs vs. Cats:

• Hypothyroidism is common.
• Hyperthyroidism is relatively uncommon.

Recognizing Issues & Treatment of Thyroid issues in Dogs:

*   Between ages 2-6
*   Weight gain, loss of energy.
*   Thyroxine Treatments.
            *   Restoring thyroid hormone will allow the animal to thrive normally.
            *   Administer thyroxine once or twice a day.

Diabetes Mellitus

• A big disease in dogs. occurs in cats.
• Deficiency of insulin: Can be either where animal doesn't produce enough insulin or the body doesn't respond to it.
  * Insulin Resistance
• People have been understanding symptoms of the disease long ago.
  * Roman Physician Celsus, described the affects 2000 years ago.
  * English Physician Willis recognize the sweetness that urine from people of Diabetes carries 300 years ago.
  * Landmark Event: In 1921 a scientists were able to crystallize insulin.
  * 1950's - Structure of Insulin was reduced.
  * 1980's - using Insulin produced by Recombinant DNA methods.

Symptoms:

• High levels of Glucose in bloodstream.
• Spill over of glucose into urine.
• Water takes the Glucose and excessive urination occurs (Polyuria)
• Excessive thirst (Polydipsia)
• Animal Eats all the time
• Untreated Diabetics and Animals typically start losing weight
• Diabetes can give dogs Cataracts.

How to recognize the disease in Dogs

• This is a disease of older dogs. All the insulin is usually gone in dogs that are diabetic.
• Approaches usually used in cats/ and humans do not work with Dogs. Reason being is because usually no Insulin production is present in dogs.

How Insulin Works

• How Normal Animals respond to insulin:
  * Pancreas produces insulin and allows the insulin receptors on cell surface.
  * Channels are activated and glucose is transported to cells.
• Type I: Animals aren't making enough insulin, therefore insulin receptors don't activate Glucose transport via GLU - four.
• Type II: Producing Insulin is okay, problem is with the insulin receptor responding normally. Similar transport process is caused leading to glucose not entering cell.

Adrenal Gland

Structure:

• Adrenal Cortex - Group of Hormones the Medulla.
• Aldosterone: mineral- corticoid.
• Cortisol is a glucorticoid.

Function:

• Important effects Metabolically.

• Involved in stress Responses.

• Glucocorticoids affect Immune system and Skeleton.

• The body is affected Deleteriously. It causes breakdown of Adipose and Muscles.

• Diseases
  * Cushing's Disease: hyperadrencordtism.
  * increased production of cortisol (typically)
  * High blood glucose causes a stress on the endocrine system and can lead to Diabetes Mellitus.
  * It is important to keep blood levels of Glucose elevated because they provide for the brain.
  * Symptoms include Polydipsia, and Polyuria as well and changes in the abdomen.
  * Linked To other Diseases.
  * The disease is usually a slow process and will get worst. Treated effectively typically.
  ### Types of Cushing's Disease:

         *The Pituitary  is the problem usually. Making it the most common form, 80 \%*.
          *   It drives high levels of production of glucocorticoids from the adrenal gland which drives Cushing's.
      *The adernal gland is usually the source of the problem as a adrenal tumor*. It can be malignant or non - malignant. 
  
      *   Treatment  to tumors include a process of removing the malignant tumor.
  
       *   Lisodrine - A drug used because its a chemical cousin to DDT. Concentrates in the adrenal gland killing a number of the cells.
                   * Given too little or too many can be issues. Over the past decade this form of curing the disease has changed.
                   *Trilastane : A drug used on approved in \frac{02}{2008}. This drug suppresses glucocortoid production by the dog and be used for possible animal use as well. 
  
        *   Addison's Disease: (hypoadrenocortism).
             -Relatively uncommon in dogs.  Requires killing a lot of adrenal cells before issues in bodily functions. 
         -Treated effectively via Hormone replacement. A small percentage of adrenal cells need to work for function of body. 
  
      Hopefully I have have given you insights on the endocrine system, thank you.