Endocrine Glands and Thyroid Gland Overview

Endocrine Glands and Tissues

• The endocrine system includes several glands and tissues
• These are the pineal gland, pituitary gland (anterior and posterior), thyroid gland, adrenal glands, pancreas, and gonads (testes and ovaries)

Thyroid Gland

• Butterfly-shaped, located in front of the trachea, below the larynx
• Releases two hormones:
  • Thyroid hormone (TH): regulates metabolic rate and heat production. Most body cells are target cells. It increases energy use, glucose and lipid use, and promotes cellular ATP production and protein synthesis.
  • Calcitonin: regulates calcium levels in the blood.

Thyroid Hormone (TH)

• Two forms: T3 and T4
• Differ in the number of iodine atoms (3 vs 4)
• T3 and T4 are hydrophobic and bound to blood proteins
• Released T3/T4 enters bloodstream and cells; T3 binds to intracellular receptors, triggering a nuclear response, for example, affecting gene expression
• Iodine from the diet is necessary to produce T3 and T4
• Salt is often iodized
• Hypothalamus ultimately controls TH release; use of negative feedback
• TH levels are maintained by a negative feedback loop
• Checking TRH, TSH, and TH levels is important when metabolic imbalances are suspected

Regulation of TH Secretion

• TRH (thyrotropin-releasing hormone) from the hypothalamus and TSH (thyroid-stimulating hormone) from the anterior pituitary regulate TH secretion
• Elevated T3 in the blood inhibits the release of both TRH and TSH

Thyroid Disorders

• Hypothyroidism: Reduced levels of TH, leading to a slower metabolism
• Causes can include problems with the hypothalamus, anterior pituitary, or thyroid gland
• Symptoms include fatigue, memory impairment, depression, swollen face, weight gain, dry skin, muscle cramps, shaggy hair, and hair loss.
• Could be due to Hashimoto's disease where the immune system attacks the thyroid
• Hypothyroidism is treatable
• Hyperthyroidism: Elevated levels of TH, leading to a faster metabolism
• Causes can include problems with the hypothalamus, anterior pituitary, or thyroid gland
• Symptoms include nervousness, irritability, insomnia, depression, weight loss, strong hunger, diarrhea, fragile fingernails, shaking hands, warm moist skin, increased body temperature.
• Could be Graves' Disease where antibodies mimic TSH and stimulate the thyroid inappropriately

4. Adrenal Glands

• Small glands located on top of each kidney
• Two regions:
  • Adrenal cortex: produces 20 steroid hormones (hydrophobic), including male and female sex hormones, cortisol
  • Adrenal medulla: produces short-term stress hormones (e.g., epinephrine, norepinephrine)

Stress Responses

• Enables the body to adapt by mobilizing nutrients, increasing metabolic rates, increasing nutrient and oxygen delivery, and suppressing non-essential functions
• Short-term stress (e.g., exercise, excitement) stimulates the "fight or flight" response, increasing heart rate, breathing, oxygen, glucose supply, skeletal muscle readiness, and blood pressure and suppressing non-essential organs.
• Long-term stress causes the hypothalamus to release hormone CRH which causes the anterior pituitary to stimulate the adrenal cortex, resulting in mineralcorticoid and glucocorticoid production.

Cortisol

• Released daily, part of the circadian rhythm
• Involved in long-term stress responses
• Mobilizes stored nutrients (carbs, fats, proteins) for energy
• Suppresses immune system function

5. Pancreas

• Located on the back of the stomach
• Produces two antagonistic hormones:
  • Glucagon: increases blood glucose levels by stimulating glycogen breakdown in the liver and releasing glucose into the bloodstream
  • Insulin: decreases blood glucose levels by inhibiting glycogen breakdown in the liver, stimulating muscle and adipose tissue uptake

Diabetes Mellitus

• High blood glucose levels
• Type I - Early onset; autoimmune condition where the body destroys insulin-producing cells in the pancreas, requiring insulin injections.
• Type II - Later onset; body becomes insensitive to insulin, often related to diet, lifestyle, and obesity
• Symptoms include- Extreme thirst and hunger; frequent urination; unexplained weight loss; fatigue, blurred vision, nausea and vomiting
• Untreated diabetes can lead to long-term complications like kidney disease, nerve damage, cardiovascular issues and blindness

6. Gonads (Testes and Ovaries)

• Testes (males) and ovaries (females)
• Produce gametes and sex hormones
• HPG Axis: Hypothalamus, pituitary gland and Gonads are stimulated by Gonadotrophin releasing hormone from the hypothalamus; Anterior pituitary releasing LH and FSH which stimulate the gonads.

Testosterone

• Produced in the testes
• Important for sperm production
• Promotes male secondary sex characteristics; development of male genitals; maintenance of male reproductive structures ; sexual drive.

Estrogen

• Found in both males and females but higher in females (ovaries)
• Important for menstrual cycle
• Development and maintenance of female reproductive structures
• Development of secondary sexual characteristics, maintaining bone density, and prepares for pregnancy