What is the nature of the thyroid?

What is Nature Thyroid?

Nature Thyroid is not a standard medical term or recognized pharmaceutical product in the provided evidence. However, based on the context of thyroid-related information, you may be asking about either: (1) the natural/physiological nature of thyroid hormones, or (2) natural desiccated thyroid products (which are sometimes referred to colloquially). I'll address the physiological nature of thyroid hormones, as this is what the evidence supports.

Biosynthesis of Thyroid Hormones

Thyroid hormones are unique among hormones because they are synthesized from amino acids embedded within a large protein precursor called thyroglobulin, rather than being made from free amino acids. 1

• Thyroglobulin is a massive glycoprotein comprised of two identical subunits of 330,000 Da each 1
• Tyrosine residues within thyroglobulin are iodinated by thyroid peroxidase enzyme 1
• The aromatic rings of proximal tyrosine residues are diiodinated, and thyroid peroxidase subsequently carries out oxidative coupling of two iodinated tyrosine residues 1
• Hydrolysis then releases T4 (3',5'-tetraiodothyronine) and small amounts of T3 (3',5-triiodothyronine) from the incompletely iodinated tyrosine 1

Chemical Structure and Composition

Thyroid hormones are iodinated derivatives of the amino acid tyrosine. 2

• Levothyroxine (T4) has the empirical formula C₁₅H₁₀I₄NNaO₄•H₂O with a molecular weight of 798.86 (anhydrous) 2
• T4 contains four iodine atoms, while T3 contains three iodine atoms 2, 3
• Synthetic T4 (levothyroxine) is chemically identical to that produced in the human thyroid gland 2

Regulation of Production

TSH (thyroid-stimulating hormone) from the pituitary gland is the primary regulator of thyroid hormone production. 4, 5

• TSH stimulates iodide uptake into thyroid cells 4
• TSH activates thyroid peroxidase enzyme 4
• TSH regulates oxidation and organification of iodide 4
• TSH controls secretion of T4 and T3 into circulation 4
• TSH influences post-translational modifications of thyroglobulin 4

Nutritional Requirements

Adequate iodine intake is essential for proper thyroid hormone synthesis, with daily requirements of 150 μg/day in adults. 4

• Iron is crucial for proper thyroid function, as iron deficiency impairs thyroid metabolism 4
• Selenium is required for the deiodination of T4 to T3 via the selenoenzyme Type 1 5'-deiodinase 4, 6
• Zinc and copper also play roles in thyroid hormone metabolism, though less well-defined 6

Physiological Actions

The physiological actions of thyroid hormones are produced predominantly by T3, with approximately 80% of circulating T3 derived from peripheral conversion of T4. 2

• Thyroid hormones exert their effects through control of DNA transcription and protein synthesis 2
• T3 and T4 diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA 2
• This hormone-nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins 2

Metabolic and Cardiovascular Effects

T3 regulates multiple metabolic processes including glucose metabolism, lipid metabolism, and cardiovascular function. 5

• T3 regulates daily rhythms in glucose metabolism, with faster glucose clearance during the early active phase 5
• T3 controls lipolysis and fatty acid oxidation during fasting states 5
• T3 increases the force and speed of systolic contraction and diastolic relaxation in cardiac tissue 5
• T3 decreases systemic vascular resistance and increases resting heart rate 5
• T3 increases left ventricular contractility and enhances isovolumic ventricular relaxation 5

Pharmacokinetics

T4 has a half-life of 6-7 days in euthyroid patients, while T3 has a half-life of ≤2 days. 2

• Circulating thyroid hormones are greater than 99% bound to plasma proteins (TBG, TBPA, and albumin) 2
• Only unbound hormone is metabolically active 2
• The liver is the major site of degradation for both T4 and T3 2
• Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3) 2
• Thyroid hormones are primarily eliminated by the kidneys, with approximately 20% eliminated in the stool 2

Clinical Significance

Thyroid disease affects nearly every organ system in the body, with hypothyroidism resulting from insufficient thyroid hormone production and hyperthyroidism from excess production. 7, 8

• Hypothyroidism causes decreased metabolism with symptoms including fatigue and weight gain 7
• Hyperthyroidism increases metabolic rate with symptoms including anxiety and tremor 7
• Both conditions are associated with significant cardiovascular, skeletal, and neurological effects 8