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Related Subjects: |Thyrotoxicosis and Hyperthyroidism |Thyroid Storm - Thyrotoxic crisis |Graves Disease (Thyrotoxicosis) |Amiodarone and Thyroid disease |Thyroid Surgery (Thyroidectomy) |Hypothyroidism |Hashimoto's thyroiditis |DeQuervain's thyroiditis |Subacute Thyroiditis |Thyroid nodule |Congenital Hypothyroidism |Thyroid Function Tests and antibodies |Post partum thyroiditis |Sick Euthyroid Syndrome |Thyroid Exam (OSCE) |Thyroid Gland anatomy and Physiology |Thyroid Cancer

The thyroid trap is an ATP driven Iodide pump found in the basal plasma membrane of the follicular cells which transports iodine against its concentration gradient into the colloid. Thyroglobulin rich in tyrosine residues is in the colloid and is available for iodination. Iodination of thyroglobulin is by thyroid peroxidase.

Overview of Thyroid Physiology

The thyroid gland is a vital endocrine organ located in the neck, responsible for producing hormones that regulate metabolism, growth, and development. The primary hormones produced by the thyroid gland are thyroxine (T4) and triiodothyronine (T3).

Anatomy of the Thyroid Gland

• Located in the anterior neck, just below the larynx.
• Consists of two lobes connected by an isthmus.
• Composed of follicles filled with colloid, which contains thyroglobulin, the precursor of thyroid hormones.

Synthesis of Thyroid Hormones

• Iodine Uptake:
  • Iodine is actively transported into thyroid follicular cells from the bloodstream via the sodium-iodide symporter.
• Thyroglobulin Production:
  • Thyroglobulin, a large glycoprotein, is synthesized in the follicular cells and secreted into the colloid.
• Iodination and Coupling:
  • Within the colloid, iodine binds to tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
  • Two DIT molecules combine to form T4, while one MIT and one DIT combine to form T3.
• Storage and Release:
  • Thyroglobulin with attached T3 and T4 is stored in the colloid.
  • Upon stimulation by thyroid-stimulating hormone (TSH), thyroglobulin is endocytosed back into the follicular cells, where proteolysis releases T3 and T4 into the bloodstream.

Regulation of Thyroid Hormone Production

• Hypothalamic-Pituitary-Thyroid Axis:
  • The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to release TSH.
  • TSH stimulates the thyroid gland to produce and release T3 and T4.
  • Negative feedback: High levels of T3 and T4 inhibit the release of TRH and TSH, maintaining hormone balance.

Actions of Thyroid Hormones

• Metabolism:
  • Increase basal metabolic rate (BMR) by stimulating the production of ATP and heat.
  • Enhance carbohydrate and lipid metabolism.
• Growth and Development:
  • Essential for normal growth and development, particularly of the central nervous system in infants and children.
  • Promote protein synthesis and bone growth.
• Cardiovascular System:
  • Increase heart rate, cardiac output, and blood flow by enhancing the responsiveness of the heart to catecholamines.
• Central Nervous System:
  • Influence mood, cognition, and overall mental function.

Thyroid Function Tests

• Serum TSH: Primary screening test for thyroid function. Elevated in hypothyroidism and decreased in hyperthyroidism.
• Free T4: Measures the unbound thyroxine level in the blood, providing a clear indicator of thyroid hormone production.
• Free T3: Measures the unbound triiodothyronine level in the blood, often used when hyperthyroidism is suspected.
• Thyroid Antibodies: Detect autoimmune thyroid conditions, such as Hashimoto's thyroiditis (anti-TPO antibodies) and Graves' disease (TSI).

Clinical Relevance

• Hypothyroidism:
  • Caused by insufficient production of thyroid hormones.
  • Symptoms: Fatigue, weight gain, cold intolerance, constipation, and depression.
  • Common causes: Hashimoto's thyroiditis, iodine deficiency, and certain medications.
• Hyperthyroidism:
  • Caused by excessive production of thyroid hormones.
  • Symptoms: Weight loss, heat intolerance, palpitations, anxiety, and tremors.
  • Common causes: Graves' disease, toxic multinodular goiter, and thyroiditis.

Summary

The thyroid gland plays a critical role in regulating metabolism, growth, and development through the production of thyroid hormones T3 and T4. These hormones are synthesized from iodine and tyrosine, stored in the colloid, and released into the bloodstream under the regulation of the hypothalamic-pituitary-thyroid axis. Thyroid function tests help diagnose and manage disorders such as hypothyroidism and hyperthyroidism, which can significantly impact overall health.