Lecture on Thyroid Gland and Metabolism

Introduction

• The thyroid gland is a small gland (20-30 grams) with significant effects on metabolism.
• Dysfunction in hormone secretion affects metabolism drastically:
  • Hyposecretion can decrease metabolism by 40-50%.
  • Hypersecretion can increase metabolism by 60-100%.

Thyroid Gland Overview

• Located below the larynx (Adam's apple), adjacent to the trachea.
• Made up of follicular cells responsible for producing and secreting thyroid hormones.

Activation and Regulation

• Hypothalamus-Pituitary-Thyroid Axis: Similar to a thermostat controlling a furnace.
  • Hypothalamus secretes TRH (Thyrotropin-releasing hormone).
  • TRH stimulates the pituitary gland to release TSH (Thyroid-stimulating hormone).
  • TSH stimulates the thyroid gland to produce T3 and T4 hormones.

Thyroid Hormones

• T3 (Tri-iodothyronine): Contains 3 iodine atoms.
• T4 (Tetra-iodothyronine): Contains 4 iodine atoms.
• Iodine is necessary to synthesize these hormones; deficiency leads to thyroid issues.
  • Source of iodine: Iodized salt.
• Clinical Tests: Typically TSH (pituitary) and T4 (thyroid).

Metabolic Effects

• Thyroid hormones impact multiple bodily functions by increasing metabolism:
  • Increase size and number of mitochondria (ATP production).
  • Enhance carbohydrate metabolism (glycolysis, gluconeogenesis, absorption).
  • Enhance fat metabolism (mobilize free fatty acids, reduce cholesterol and triglycerides).

Impacts of Hypothyroidism and Hyperthyroidism

• Hypothyroidism (low thyroid hormone):

  • Slowed metabolism: Weight gain, high cholesterol, low energy.
  • Cardiac effects: Weak heart contractility, low heart rate.
  • Muscular effects: Sluggish muscle contraction.
  • Central nervous effects: Somnolence, lethargy.
  • Sexual effects: Decreased libido, menstrual irregularities.

• Hyperthyroidism (high thyroid hormone):

  • Increased metabolism: Weight loss, low cholesterol.
  • Cardiac effects: High heart rate, potential myocardial failure.
  • Muscular effects: Muscle weakening due to protein breakdown.
  • Central nervous effects: Anxiety, agitation.
  • Sexual effects: Impotence in males, menstrual irregularities in females.

Regulation and Feedback

• Feedback mechanism similar to a household thermostat:
  • Hypothalamus and pituitary detect low/high hormone levels and regulate secretion accordingly to maintain balance.

Clinical Scenarios

• Primary Hypothyroidism: Low T4, high TSH. Issue with the thyroid not responding to TSH.
• Secondary Hypothyroidism: Low T4, low TSH. Issue with the pituitary not secreting TSH.
• Hyperthyroidism via Thyroid (Primary): High T4, low TSH. Thyroid overacting independently of TSH.
• Hyperthyroidism via Pituitary (Secondary): High T4, high TSH. Pituitary over secreting TSH due to possible tumor.

Conclusion

• Importance of maintaining balanced hormone levels (Goldilocks principle).
• Regular diagnostic tests focus mainly on TSH and T4 levels.
• The significance of these hormones on overall health and metabolism.