Thyroid Gland and Hormones

Thyroid Gland

• Secretes thyroid hormones
  • Thyroxin (T4)
  • Triiodothyronine (T3)
  • Thyrocalcitonin (Ca lowering hormone)
• Follicular cells produce T4 and T3
• Parafollicular cells produce Thyrocalcitonin

Thyroid Hormones

• T4 and T3 contain iodine
• Similar structure: two tyrosine amino acids linked together with 4 or 3 iodine atoms respectively

Functions of Thyroid Hormones

• Metabolic functions

  • Calorigenic action
    • Increases mitochondria size and number in cells
    • Increases ATP formation
    • Increases basal metabolic rate and energy consumption
  • Metabolic processes
    • Carbohydrates
      • Increases glucose levels and utilization in blood
      • Increases glycolysis, glycogenolysis, and gluconeogenesis
    • Lipids:
      • Powerful lipolytic action on fat stores
      • Depletes fat stores
      • Decreases body weight
    • Protein
      • Normal levels: Stimulate protein synthesis (anabolic)
      • High levels: Cause protein catabolism

• Growth

• Effect on body systems

  • Cardiovascular system:
    • Increases heart rate
    • Increases strength of contractility
    • Increases systolic blood pressure
  • Respiratory system:
    • Increases rate and depth of respiration
  • Gastrointestinal tract:
    • Increases appetite
    • Increases food intake
    • Increases secretion and motility
  • Physical, Mental, and Sexual systems - further detail not provided

Thyroid Gland

• The thyroid gland secretes thyroid hormones.
• Thyroid hormones: Thyroxin (T4), Triiodothyronine (T3), and Thyrocalcitonin
  • T4 and T3 are secreted by follicular cells and contain iodine.
  • Thyrocalcitonin is secreted by parafollicular cells and lowers calcium levels.
• T4 and T3 are similar in structure, with two tyrosine amino acids linked together and either 4 or 3 atoms of iodine, respectively.

Functions of Thyroid Hormones

• Metabolic functions:

  • Calorigenic action: Increases the size and number of mitochondria within cells, leading to increased ATP formation and basal metabolic rate.
  • Metabolic processes:
    • Carbohydrate: Increases glucose levels and utilization in the blood by increasing glycolysis, glycogenolysis, and gluconeogenesis.
    • Lipids: Promotes lipolysis (fat breakdown) and depletion of fat stores.
    • Proteins: At normal levels, thyroid hormones stimulate protein synthesis (anabolic). High levels cause protein catabolism (breakdown).

• Growth: Promotes normal growth and development, particularly in the brain and skeletal system.

• Effect on body systems:

  • Cardiovascular system: Increases heart rate, strength of contraction, and systolic blood pressure.
  • Respiratory system: Increases rate and depth of respiration.
  • Gastrointestinal tract: Increases appetite, food intake, secretion, and motility.

Grave's Disease

• An autoimmune disorder causing hyperthyroidism
• Thyroid Stimulating Immunoglobulin (TSI) mimics TSH, stimulating thyroid hormone production
• TSI results in elevated T3 and T4 levels
• Genetic predisposition associated with HLA alleles like HLA-DR3
• Family history increases risk
• Environmental triggers like stress, infection, and hormonal changes can precipitate the disease
• Lymphocytic infiltration disrupts normal thyroid function

Symptoms

• Hyperactivity, anxiety, and weight loss despite increased appetite
• Heat intolerance, excessive sweating, and tremors
• Muscle weakness
• Exophthalmos (eye protrusion), eye irritation, and vision changes
• Thyroid dermopathy (thickened skin, often on shins)
• Palpitations and tachycardia

Diagnosis

• Clinical evaluation of symptoms and medical history
• Low TSH levels due to high T3 and T4
• Elevated T3 and T4 levels
• Increased radioactive iodine uptake indicating thyroid gland hyperactivity
• Positive Thyroid Stimulating Immunoglobulin (TSI) and thyroid peroxidase antibody tests confirm autoimmune activity
• Ultrasound may show thyroid gland enlargement

Causes of Myxedema

• Myxedema develops due to a lack of thyroid hormones.
• While multiple factors can lead to this, Hypothyroidism, a condition where the thyroid gland does not produce sufficient hormones, is often the root cause.
• Autoimmune disorders, such as Hashimoto's thyroiditis, can damage the thyroid gland, leading to a decrease in hormone production.
• Iodine is crucial for producing thyroid hormones; therefore, iodine deficiency can prevent the synthesis of these hormones.
• Certain medications like lithium and amiodarone can interfere with thyroid function, potentially causing myxedema.
• Surgical removal of the thyroid gland, whether partially or fully, can lead to hormone deficiencies, increasing the risk of developing myxedema.
• Radiation therapy, particularly for cancers in the head, neck, or thyroid, can damage thyroid tissue, interfering with hormone production.
• Some individuals are born with a thyroid gland that does not function properly, increasing their susceptibility to myxedema.

Symptoms of Myxedema

• Myxedema affects multiple bodily systems and results in a range of noticeable symptoms.
• Fatigue, extreme tiredness, and lack of energy are common.
• Individuals with myxedema often experience sensitivity to cold temperatures.
• Due to slow metabolism, weight gain is frequently observed.
• Dry skin and hair are characteristic symptoms.
• Puffiness, particularly in the face, is a common symptom and is referred to as "myxedema."
• Myxedema can lead to depression, mood swings, memory loss, and difficulty concentrating.
• Slower digestive processes often result in constipation.
• Muscle weakness, especially in the lower extremities, can be experienced.
• A decreased heart rate, referred to as bradycardia, is another common symptom.
• Women may experience menstrual irregularities or issues with fertility.

Explore the intricacies of the thyroid gland, its hormones, and their significant metabolic functions. This quiz covers key aspects such as the production of thyroxine (T4), triiodothyronine (T3), and the regulatory role of these hormones in metabolism. Test your knowledge on how these hormones affect energy consumption and metabolic processes.