Thyroid Hormone Associated Diseases and Disorders
Physiological functions of thyroid hormones (primarily Tβ [thyroxine] and Tβ [triiodothyronine]):
πΉ Thyroid hormones overview
- Tβ (thyroxine) is the main hormone secreted by the thyroid but mostly a prohormone.
- Tβ (triiodothyronine) is the biologically active form, produced by peripheral deiodination of Tβ.
- Both act via thyroid hormone receptors (TRs) in the nucleus, regulating gene transcription across almost all tissues.
πΉ Physiological functions of thyroid hormones
| System / Tissue | Physiological function | Mechanism / Effect |
| β‘ Metabolism | β Basal metabolic rate (BMR) | β Mitochondrial activity, β NaβΊ/KβΊ-ATPase activity, β oxygen consumption, heat production (thermogenesis) |
| π§ CNS development & function | Essential for normal brain maturation | Promotes neuronal proliferation, differentiation, synaptogenesis, myelination (especially fetal & early postnatal) |
| 𦴠Growth and skeletal development | Supports linear growth | Synergizes with growth hormone (GH) and IGF-1; stimulates chondrocytes and osteoblasts |
| β€οΈ Cardiovascular system | β Heart rate, cardiac output, contractility | Upregulates Ξ²-adrenergic receptors; β myocardial contractility and diastolic relaxation |
| π½οΈ Carbohydrate metabolism | β Glucose turnover | β Gluconeogenesis, β glycogenolysis, β intestinal glucose absorption |
| π©Έ Lipid metabolism | β Lipolysis and cholesterol metabolism | β LDL receptor expression β β serum cholesterol; β fatty acid oxidation |
| 𧬠Protein metabolism | Stimulates protein turnover | Protein synthesis and degradation; net effect depends on thyroid status |
| π Neuromuscular system | Maintains normal reflexes and muscle tone | Modulates contractile proteins and motor neuron activity |
| π§ Behavioral / cognitive | Regulates alertness, mood | Essential for mental well-being; low thyroid = sluggishness, depression; high thyroid = anxiety, restlessness |
| π‘οΈ Thermoregulation | Generates heat | Via β mitochondrial uncoupling and energy turnover |
| π‘οΈ Sympathetic nervous system modulation | Enhances sensitivity to catecholamines | Potentiates βfight or flightβ responses (tachycardia, tremor) |
πΉ Special roles during development
β Fetal and early postnatal period:
- Normal thyroid hormone levels are critical for brain development and skeletal maturation.
- Deficiency in this period β cretinism (congenital hypothyroidism): irreversible intellectual disability, growth retardation, deafness.
πΉ 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.
- Tβ is the main active form at the cellular level, even though the thyroid gland mainly secretes Tβ.