What is the role of thyroid hormones in the body?

Role of Thyroid Hormones in the Body

Thyroid hormones are essential regulators that exert profound influence on every organ system in the body through control of DNA transcription and protein synthesis, with particularly crucial roles in metabolism, cardiovascular function, and development of the central nervous system. 1, 2

Primary Functions of Thyroid Hormones

• Thyroid hormones control metabolism by enhancing oxygen consumption in most tissues, increasing basal metabolic rate, and regulating the metabolism of carbohydrates, lipids, and proteins 2
• T3 regulates daily rhythms in glucose metabolism, with faster glucose clearance occurring during the early active phase 3
• T3 controls lipolysis and fatty acid oxidation during fasting states, influencing serum triglyceride levels and overall lipid metabolism 3
• Thyroid hormones are crucial for proper development, particularly of the central nervous system 2

Mechanism of Action

• Thyroid hormones (T3 and T4) exert their physiologic actions through control of DNA transcription and protein synthesis by diffusing into cell nuclei and binding to thyroid receptor proteins attached to DNA 1
• This hormone-nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins 1
• The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues 1

Cardiovascular Effects

• T3 increases the force and speed of systolic contraction and the speed of diastolic relaxation in cardiac tissue 3, 4
• T3 decreases systemic vascular resistance, including coronary vascular tone 4
• T3 increases resting heart rate and left ventricular contractility 4
• T3 enhances isovolumic ventricular relaxation, improving cardiac function 4

Regulation and Production

• Thyroid hormone production is primarily regulated by thyroid stimulating hormone (TSH) from the pituitary gland 5
• TSH controls multiple aspects of thyroid hormone synthesis, including stimulation of iodide uptake, activation of thyroid peroxidase enzyme, regulation of oxidation and organification of iodide, and control of secretion of T4 and T3 into circulation 5
• T3 participates in TSH secretion regulation through negative feedback 3
• Adequate iodine intake (150 μg/day in adults) is essential for proper thyroid hormone synthesis 5

Pharmacokinetics and Metabolism

• Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (TBA) 1
• T4 has a longer half-life (6-7 days) compared to T3 (≤2 days) due to higher protein binding (99.96% vs 99.5%) 1, 2
• The major pathway of thyroid hormone metabolism is through sequential deiodination, with approximately 80% of circulating T3 derived from peripheral T4 by monodeiodination 1
• The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at additional sites including the kidney and other tissues 1

Nutritional Requirements for Thyroid Function

• Iron and selenium are crucial for proper thyroid function 5
• Iron deficiency can impair thyroid metabolism 5, 3
• Selenium is required for the deiodination of T4 to T3 via the selenoenzyme Type 1 5'-deiodinase 5, 4

Clinical Implications

• Abnormal thyroid hormone levels can lead to thyroid dysfunction, manifesting as either hypothyroidism (insufficient thyroid hormones) or hyperthyroidism (excess thyroid hormones) 3, 4
• Hypothyroidism results in decreased metabolic rate, while hyperthyroidism increases metabolic rate 4
• Atrial arrhythmias, particularly atrial fibrillation, can occur in elderly patients with thyroid hormone overtreatment 1

Pitfalls in Thyroid Hormone Management

• Many drugs and foods can affect thyroid hormone absorption and metabolism 1
• Dietary fiber decreases bioavailability of T4 1
• Thyroid hormone absorption may decrease with age 1
• Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone, with only unbound hormone being metabolically active 1