Thyroid Hormone Associated Diseases and Disorders
Physiological functions of thyroid hormones (primarily T₄ [thyroxine] and T₃ [triiodothyronine]):
🔹 Thyroid hormones overview
- T₄ (thyroxine) is the main hormone secreted by the thyroid but mostly a prohormone.
- T₃ (triiodothyronine) is the biologically active form, produced by peripheral deiodination of T₄.
- Both act via thyroid hormone receptors (TRs) in the nucleus, regulating gene transcription across almost all tissues.
🔹 Physiological functions of thyroid hormones
| System / Tissue | Physiological function | Mechanism / Effect |
| ⚡ Metabolism | ↑ Basal metabolic rate (BMR) | ↑ Mitochondrial activity, ↑ Na⁺/K⁺-ATPase activity, ↑ oxygen consumption, heat production (thermogenesis) |
| 🧠 CNS development & function | Essential for normal brain maturation | Promotes neuronal proliferation, differentiation, synaptogenesis, myelination (especially fetal & early postnatal) |
| 🦴 Growth and skeletal development | Supports linear growth | Synergizes with growth hormone (GH) and IGF-1; stimulates chondrocytes and osteoblasts |
| ❤️ Cardiovascular system | ↑ Heart rate, cardiac output, contractility | Upregulates β-adrenergic receptors; ↑ myocardial contractility and diastolic relaxation |
| 🍽️ Carbohydrate metabolism | ↑ Glucose turnover | ↑ Gluconeogenesis, ↑ glycogenolysis, ↑ intestinal glucose absorption |
| 🩸 Lipid metabolism | ↑ Lipolysis and cholesterol metabolism | ↑ LDL receptor expression → ↓ serum cholesterol; ↑ fatty acid oxidation |
| 🧬 Protein metabolism | Stimulates protein turnover | Protein synthesis and degradation; net effect depends on thyroid status |
| 🛌 Neuromuscular system | Maintains normal reflexes and muscle tone | Modulates contractile proteins and motor neuron activity |
| 🧠 Behavioral / cognitive | Regulates alertness, mood | Essential for mental well-being; low thyroid = sluggishness, depression; high thyroid = anxiety, restlessness |
| 🌡️ Thermoregulation | Generates heat | Via ↑ mitochondrial uncoupling and energy turnover |
| 🛡️ Sympathetic nervous system modulation | Enhances sensitivity to catecholamines | Potentiates “fight or flight” responses (tachycardia, tremor) |
🔹 Special roles during development
✅ Fetal and early postnatal period:
- Normal thyroid hormone levels are critical for brain development and skeletal maturation.
- Deficiency in this period → cretinism (congenital hypothyroidism): irreversible intellectual disability, growth retardation, deafness.
🔹 Key summary points
- Thyroid hormones regulate metabolism, growth, development, thermoregulation, and cardiovascular dynamics.
- They are global regulators, affecting nearly every tissue and organ in the body.
- T₃ is the main active form at the cellular level, even though the thyroid gland mainly secretes T₄.