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Overview of Iodine Physiology

Iodine is an essential trace element necessary for the synthesis of thyroid hormones, which are critical for regulating metabolism, growth, and development. The human body cannot produce iodine, so it must be obtained from the diet.

Sources of Iodine

• Iodine is naturally found in various foods:
  • Seafood, such as fish, shellfish, and seaweed, are rich sources of iodine.
  • Dairy products, eggs, and some fruits and vegetables also contain iodine.
  • Iodized salt is a significant source of iodine in many countries.

Absorption and Transport

• Iodine is absorbed primarily in the stomach and small intestine.
  • Once absorbed, iodine is transported in the bloodstream as iodide (I^-).
  • The thyroid gland actively concentrates iodide from the blood using a sodium-iodide symporter.

Role in Thyroid Hormone Synthesis

• Iodine is crucial for the synthesis of thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃):
  • Thyroid Hormone Production:
    • Iodide is oxidized to iodine by the enzyme thyroid peroxidase (TPO).
    • Iodine is then incorporated into the amino acid tyrosine within the protein thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
    • Coupling of MIT and DIT forms T₃, while coupling of two DIT molecules forms T₄.
  • Release of Thyroid Hormones:
    • Thyroglobulin is endocytosed back into thyroid follicular cells and proteolytically cleaved to release T₃ and T₄ into the bloodstream.

Functions of Thyroid Hormones

• Thyroid hormones have wide-ranging effects on the body:
  • Metabolism:
    • Increase basal metabolic rate (BMR) by stimulating the production and use of ATP.
    • Regulate the metabolism of carbohydrates, fats, and proteins.
  • Growth and Development:
    • Essential for normal growth and development, particularly of the brain and skeletal system during fetal and neonatal periods.
  • Thermoregulation:
    • Help maintain body temperature by regulating heat production.
  • Cardiovascular System:
    • Increase heart rate, cardiac output, and blood flow to tissues.
  • Nervous System:
    • Influence the development and function of the nervous system.

Regulation of Thyroid Function

• Thyroid function is regulated by the hypothalamic-pituitary-thyroid (HPT) axis:
  • Thyrotropin-Releasing Hormone (TRH):
    • Produced by the hypothalamus, stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH).
  • Thyroid-Stimulating Hormone (TSH):
    • Produced by the anterior pituitary, stimulates the thyroid gland to produce and release T₃ and T₄.
  • Negative Feedback:
    • Elevated levels of T₃ and T₄ inhibit the release of TRH and TSH, maintaining hormone balance.

Clinical Relevance

• Iodine Deficiency:
  • Leads to inadequate thyroid hormone production, causing hypothyroidism and goiter (thyroid enlargement).
  • Severe deficiency during pregnancy can result in cretinism in infants, characterized by severe intellectual and developmental delays.
  • Treatment involves iodine supplementation and, if necessary, thyroid hormone replacement therapy.
• Hypothyroidism:
  • Symptoms: Fatigue, weight gain, cold intolerance, constipation, dry skin, and depression.
  • Causes: Iodine deficiency, autoimmune thyroiditis (Hashimoto's disease), and certain medications.
  • Treatment: Thyroid hormone replacement therapy.
• Hyperthyroidism:
  • Symptoms: Weight loss, heat intolerance, anxiety, palpitations, and increased appetite.
  • Causes: Graves' disease, toxic adenoma, and thyroiditis.
  • Treatment: Antithyroid medications, radioactive iodine therapy, or surgery.
• Thyroid Cancer:
  • Diagnosis: Often detected through thyroid nodules, confirmed by ultrasound and fine-needle aspiration biopsy.
  • Treatment: Surgery, radioactive iodine therapy, and thyroid hormone suppression therapy.

Summary

Iodine is an essential trace element crucial for the synthesis of thyroid hormones, which regulate metabolism, growth, and development. Its deficiency can lead to significant health issues, including hypothyroidism and developmental delays. Understanding iodine physiology is vital for maintaining thyroid health and overall well-being.