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Lecture on Thyroid Gland and Metabolism

Jun 19, 2024

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.