Thyroid Gland Function
Primary Physiological Role
The thyroid gland produces thyroid hormones (T4 and T3) that control DNA transcription and protein synthesis throughout the body, thereby regulating metabolism, growth, development, and energy homeostasis. 1, 2
Core Functions
Hormone Production and Secretion
- The thyroid synthesizes and secretes two primary hormones: thyroxine (T4) and triiodothyronine (T3), which are iodinated derivatives of the amino acid tyrosine 3, 2
- T4 is the major hormone produced by the thyroid gland, with approximately 80% of circulating T3 derived from peripheral conversion of T4 through deiodination in tissues 1, 4
- Thyroid hormone production is regulated by thyroid-stimulating hormone (TSH) from the pituitary gland, which controls iodide uptake, oxidation and organification of iodide, and secretion of hormones into circulation 5
Metabolic Regulation
- Thyroid hormones diffuse into cell nuclei and bind to thyroid receptor proteins attached to DNA, activating gene transcription and synthesis of messenger RNA and cytoplasmic proteins 1
- These hormones regulate critical metabolic functions including glucose metabolism, lipid metabolism (lipolysis and fatty acid oxidation), and overall energy expenditure 6, 7, 2
- Thyroid hormones act as counter-regulatory hormones that mobilize endogenous glucose stores and alter glucose metabolism 6
Growth and Development
- Thyroid hormones are crucial for normal growth, differentiation, and development, particularly brain development 2, 8
- Untreated congenital hypothyroidism leads to irreversible brain dysfunction (cretinism) and dwarfism 8
Circadian and Metabolic Rhythm Regulation
- The thyroid regulates multiple metronomic functions including circadian rhythms, sleep-wake cycles, and diurnal variations in metabolism 7
- TSH and free T3 show distinct circadian patterns that peak during nighttime hours 7
Essential Nutritional Requirements
- Adequate iodine intake (150 μg/day in adults) is essential for proper thyroid hormone synthesis 5
- Iron and selenium are crucial cofactors: iron deficiency impairs thyroid metabolism, while selenium is required for conversion of T4 to T3 via selenoenzyme Type 1 5'-deiodinase 5
Clinical Significance
- Thyroid dysfunction represents a continuum from asymptomatic biochemical changes to clinically symptomatic disease, manifesting as either hypothyroidism (insufficient hormone production) or hyperthyroidism (excess hormone production) 9, 5
- Disorders of the thyroid gland are among the most common endocrine conditions, with approximately 46 per 1000 persons in the United States having subclinical hypothyroidism 9