II. Embryology

  1. Thyroid Gland develops from Tongue root (foramen cecum)
  2. Formed by 5th-6th week of development

III. Anatomy

  1. Images
    1. ThyroidAnatomy.png
  2. Normal Adult Thyroid weighs 20-25g
  3. Components
    1. Two Lateral lobes
    2. Isthmus centrally connects the lobes
  4. Adjacent structures
    1. Recurrent laryngeal nerve
    2. Parathyroid Glands

IV. Physiology

  1. Images
    1. thyroidHormoneSynthesis.png
  2. Stressors stimulate increased Thyroid function
    1. Puberty
    2. Pregnancy
    3. Physiologic stress
  3. Hypothalamus
    1. Thyrotropin Releasing Hormone (TRH) released
    2. TRH passes down pituitary stalk to pituitary
  4. Anterior Pituitary
    1. Thyroid Stimulating Hormone (TSH) released
    2. TSH passes into circulation to Thyroid
  5. Thyroid
    1. Iodine is absorbed from circulation (increased by TSH)
    2. Thyroglobulin is produced, bound to Tyrosine molecules and stored within Thyroid follicles
    3. TSH stimulates synthesis of T4 and T3 from Tyrosine bound to Thyroglobulin
    4. Thyroxine or Tetraiodothyronine (T4) released into circulation
    5. Triiodthyronine (T3) is also 20-30% produced in the Thyroid (remainder is peripheral conversion of T4)
  6. Peripheral Tissues
    1. Triiodothyronine (T3) produced from T4 deiodination
  7. Circulation
    1. Only unbound Thyroid Hormones (Free T4 or T3) are metabolically active
    2. Thyroid Binding Globulin (TBG) binds T4 and T3
      1. T4 is 99.96% Protein bound in serum (<1% is Free T4, the active form)
      2. T3 is 99.7% Protein bound in serum
    3. T4 and T3 stimulate all metabolic processes
      1. T3 is several times more potent than T4
  8. Effects of Thyroid Hormone
    1. Triiodthyronine (T3) promotes DNA Transcription resulting in RNA Translation to enzymes and Protein synthesis
    2. Peripheral tissue oxygen utilization and basal metabolic rate increased
    3. Positive chronotropic and ionotropic cardiovascular effects, with secondary increased Cardiac Output
    4. Decreases Systemic Vascular Resistance via arteriole dilation
    5. When Thyroid Hormone is in excess (e.g. Thyroid Storm), high output Heart Failure may result
  9. Feedback Loop
    1. Increased T4 and T3 levels suppress TRH and TSH

V. Pathophysiology

VI. References

  1. Mason, Herbert and Swadron in Herbert (2019) EM:Rap C3 3(11):1-10
  2. Goldberg (2014) Physiology, Medmaster, Miami, p. 136-8

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