Endocrine System Overview and Thyroid Gland

Endocrine System Overview

• The endocrine system is a complex system of glands influencing various bodily functions through hormones.
• The lecture was presented by Dr. Yasmine Gamal Sabry.

Thyroid Gland and Hormones

• The thyroid gland secretes thyroid hormones, including thyroxin (T4), triiodothyronine (T3), and thyrocalcitonin (a calcium-lowering hormone).
• T4 and T3 are produced by follicular cells.
• Thyrocalcitonin is produced by parafollicular cells (C cells).
• T4 is primarily a transport form, converted to T3 in the periphery.
• T3 is the more active form, acting faster than T4.

Thyroid Hormone Transport and Regulation

• Most released T4 and T3 bind to plasma proteins (e.g., albumin, prealbumin, and thyroxine-binding globulin (TBG)). TBG is the most important.
• More than 99% of thyroid hormone is bound and less than 1% is free.
• Globulins have higher affinity than albumin and prealbumin for binding.

Advantages of Hormone Binding

• Acts as a physiological reservoir.
• Increases the half-life of circulating hormone.
• Free hormone is physiologically active, diffuses into tissues, and inhibits pituitary TSH.

Functions of Thyroid Hormones

• 1. Metabolic Functions:

  • A- Calorigenic action: Increases the size and number of mitochondria to increase ATP formation and basal metabolic rate and energy consumption.
  • B- Metabolic processes:
    • Carbohydrates: Increases blood glucose level and utilization by increasing glycolysis, glycogenolysis, and gluconeogenesis.
    • Lipids: Has a powerful lipolytic action, causing fat store depletion and decreasing weight.
    • Protein: At normal levels, stimulates protein synthesis; high levels lead to protein catabolism.

• 2. Growth: Essential for physical, mental, and sexual growth.

  • Physical growth: Stimulates protein synthesis and growth hormone secretion, potentiating its effects.
  • Mental growth: Crucial for CNS development, growth, and function in newborns and myelination of nerve fibers and synapse development.
  • Sexual growth: Stimulates gonadal function, essential for normal menstrual cycles and fertility.

• 3. Effect on Body Systems: Thyroid hormones influence various organs and systems of the body, including:

  • Cardiovascular: Increase in heart rate and strength of contraction; increased systolic blood pressure and respiratory rate and depth.
  • Gastrointestinal: Increased appetite, food intake, secretion, and motility.
  • Bone Marrow: Increased red blood cell synthesis.

Control of Thyroid Gland Function

• Thyroid-Stimulating Hormone (TSH, Thyrotropin): Secreted by the thyrotropes of the anterior pituitary in response to TRH (thyroid-releasing hormone) from the hypothalamus. Further regulated by negative feedback loops.
• Factors controlling TSH secretion: TRH and a negative feedback loop where the free thyroid hormone levels regulate TSH release.
• Dietary iodine intake is essential for normal thyroid function (150 micrograms/day). Sufficient iodine is present in sea food, water and soil in some areas. Low intake can result in goiter.

Abnormalities of Thyroid Hormone Secretion

• Hyperthyroidism: Increased thyroid hormone secretion, characterized by conditions such as Grave's disease, thyroiditis, and hypersecreting thyroid adenoma.

• Clinical Features of Hyperthyroidism: Intolerance to heat, Fine, Straight Hair, Bulging Eyes, Facial Flushing, Enlarged Thyroid, Tachycardia, Increased Systolic BP, Breast Enlargement, Weight Loss, Muscle Wasting, Localized Edema, Nervousness, insomnia, irritability, tremors, increased appetite, weight loss, Increased intestinal movements, hyperglycemia, decreased cholesterol, Exophthalmos.

• Diagnostic Tests for Hyperthyroidism: Measurement of free T3, T4 (high); TSH concentration in plasma (low); TSI concentration (high in Grave's disease); Increased BMR (+30 to +60%); and decreased plasma cholesterol levels.

• Treatment of Hyperthyroidism: Thyroidectomy, radioactive iodine ingestion, and anti-thyroid drugs.

• Hypothyroidism: Decreased thyroid hormone secretion.

• Causes of Hypothyroidism: Autoimmune diseases (Hashimoto's thyroiditis), irradiation, surgical removal, and inadequate iodine supply.

• Clinical Features of Hypothyroidism: Intolerance to cold, decreased Basal metabolic rate, Increased body weight, dry, scaly & yellowish skin, generalized non-pitting edema, facial puffiness, constipation, anemia, lethargy, mental sluggishness, decreased alertness, slow reflexes (delayed relaxation of tendon reflex), slow speech, husky voice, increased cholesterol, depressed sexual function & amenorrhea.

• Myxedema: A severe form of hypothyroidism (involves body swelling) characterized by decreased metabolic functions, cold intolerance, and accumulation of myxematous tissue.

• Cretinism: A congenital form of hypothyroidism, leading to mental retardation and growth abnormalities in infants if not treated early.