Thyroid hormones and disease

• Thyroid Hormones


  • ↑ BMR (e.g. ↑ consumption, energy + heat)
  • ↑ mobilization of metabolic fuels
  • ↑ food intake
  • ↑ responsiveness to catecholamines
• Action of Hypothalamus-pituitary hormone


  1. Neurotransmitters

  2. HPT axis (hypothalamus → TRH → anterior pituitary → TSH → thyroid → thyroxine)

  3. Negative feedback
• Thyroid Hormone Synthesis


  1. Iodide co-transported through sodium into follicle cells

  2. Transported to thyroglobulin

  3. Endocytosed from thyroglobulin

  4. Transported to vesicles

  5. Transported to interstitial fluid

  6. Transported to capillaries
• Thyroid Diseases


  • Hypothyroidism (↓ activity)
  • Hyperthyroidism (↑ activity)
• Thyroid Hormone Structure


  • Tyrosine
  • Thyroxine (T4)
  • T3 more active than T4
• TSH


  Major controller of thyroid cell function and growth
• Mutation in TSHR leads to thyroid diseases like hyperthyroidism (Graves' disease)
• Thyroid Hormone Synthesis


  1. Iodide co-transported with Na+

  2. Oxidised and attached to thyroglobulin

  3. Released from thyroglobulin by enzymes

  4. Passively diffused through interstitial fluid and into blood
• Feedback control of Thyroid Hormones

  1. Hypothalamus → TRH secretion

  2. Anterior pituitary → TSH secretion

  3. Thyroid gland → thyroid hormone secretion

  4. Negative feedback
• Thyroid Hormone Secretion


  • Normal daily secretion rate: T4 = 100 μg/day, T3 = 6 μg/day
  • T4 has higher protein binding and lower biological activity than T3
• Conversion of T4 to T3

  1. T4 converted to T3 in peripheral tissues

  2. T3 has higher biological activity than T4
• Deiodinase Enzymes


  • Regulate conversion of T4 to T3
  • Upregulated in hypothyroidism, downregulated in hyperthyroidism
• Thyroid Function Tests


  TSH, free T4, free T3
• Effects of Thyroid Hormones

  • Increase energy expenditure, metabolism, food intake, heart rate
  • Stimulate growth and development
• Thyroid Hormones and Catecholamines

  Thyroid hormones upregulate β-adrenergic receptors, enhancing catecholamine effects
• Thyroid hormones are required for normal brain development and function
• Thyroid hormones induce enzyme expression, protein synthesis, and basal metabolic rate
• Hypothyroidism can cause cretinism in children and myxedema in adults
• Hyperthyroidism can cause goiter, weight loss, muscle weakness, palpitations, and sweating