Main Functions of Thyroid Hormones
Thyroid hormones are primary regulators of metabolic rate, controlling energy homeostasis through their effects on carbohydrate, lipid, and protein metabolism, as well as mitochondrial function and thermogenesis. 1
Metabolic Regulation
Thyroid hormones (T3 and T4) exert their physiologic actions through control of DNA transcription and protein synthesis. They diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA, activating gene transcription and synthesis of messenger RNA and cytoplasmic proteins 2. The main metabolic functions include:
- Basal Metabolic Rate: Thyroid hormones regulate substrate utilization, control basal metabolic rate, modulate heat production, and influence oxygen consumption 1
- Carbohydrate Metabolism: Increased gluconeogenesis and enhanced glucose utilization 1
- Lipid Metabolism: Enhanced cholesterol metabolism, bile acid synthesis, stimulation of lipogenesis and lipolysis 1
- Protein Synthesis: Regulation of protein synthesis and degradation 1
Cardiovascular System Effects
Thyroid hormones have profound effects on the cardiovascular system:
- Increase heart rate and contractility
- Decrease systemic vascular resistance
- Increase cardiac output (up to 300% from euthyroid state)
- Enhance blood volume (up to 25% increase)
- Stimulate erythropoietin release 1, 3
These effects explain why hyperthyroidism can lead to heart failure, particularly in patients over 50 years or with underlying cardiac disease 3, 1.
Growth and Development
Thyroid hormones are essential for:
- Normal growth and skeletal development
- Central nervous system development, particularly during fetal and early postnatal periods
- Regulation of GH and IGF-1 effects on growth 3
Thyroid Hormone Physiology
The physiological actions of thyroid hormones are produced predominantly by T3, with approximately 80% of circulating T3 derived from peripheral conversion of T4 by deiodination 2. Key aspects of thyroid hormone physiology include:
- Production: T4 and T3 are synthesized in the thyroid gland, with iodine as a crucial component 3
- Transport: Circulating thyroid hormones are >99% bound to plasma proteins (TBG, TBPA, and albumin) 2
- Metabolism: The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring in kidney and other tissues 2
- Elimination: Primarily through the kidneys, with approximately 20% eliminated in stool 2
Regulation of Thyroid Function
Thyroid hormone production is regulated by:
- Hypothalamic-Pituitary-Thyroid Axis: TRH from hypothalamus stimulates TSH release from pituitary, which stimulates thyroid hormone production
- Negative Feedback: T3 and T4 inhibit TRH and TSH production
- Iodine Availability: Iodine is essential for thyroid hormone synthesis 3
Clinical Implications
Disruptions in thyroid hormone function lead to significant clinical consequences:
- Hyperthyroidism: Increased metabolic rate, weight loss despite increased appetite, heat intolerance, tachycardia, and potential heart failure 3
- Hypothyroidism: Decreased metabolic rate, weight gain, cold intolerance, bradycardia, and potential cognitive impairment 3
Important Considerations
- Thyroid function testing is essential in patients with unexplained metabolic derangements 1
- Selenium and iron are important cofactors for proper thyroid function 3
- Thyroid disorders can significantly impact cardiovascular health, with both hypo- and hyperthyroidism associated with increased cardiovascular risk 4
Thyroid hormones represent a critical regulatory system affecting virtually every organ system in the body, with their primary role being the regulation of metabolic processes and energy homeostasis.