Thyroid and parathyroid glands

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 Thyroid and parathyroid glands
 1. Thyroid gland

  • Location: Butterfly-shaped gland, in the anterior neck (below larynx, in front of trachea).

  • Coverings: Surrounded by the pretracheal fascia and an internal capsule.

  • Blood supply:

    • Superior thyroid artery (branch of external carotid) → supplies upper & anterior gland.

    • Inferior thyroid artery (branch of subclavian via thyrocervical trunk) → supplies posterior & inferior gland.

    • Thyroid ima artery (in some people, from brachiocephalic trunk) → supplies isthmus.

  • Venous drainage:

    • Superior & middle thyroid veins → internal jugular vein.

    • Inferior thyroid veins → brachiocephalic veins.

  • Lymph drainage: paratracheal and deep cervical lymph nodes.

  • Innervation:

    • Sympathetic: cervical sympathetic ganglia.

    • Parasympathetic: vagus nerve.

⚠️ Clinical surgical risk:

  • Recurrent laryngeal nerve runs near the inferior thyroid artery → risk of hoarseness if injured.

  • External branch of superior laryngeal nerve runs near the superior thyroid artery → risk of voice pitch changes if injured.

  • Parathyroid glands may also be damaged accidentally.

2. Thyroid gland physiology

  • Hormones:

    • Thyroid hormones (T3, T4) → control metabolism, growth, heart, CNS, bone, reproduction.

    • Calcitonin (from C cells) → lowers serum calcium by inhibiting osteoclasts. (But physiological role is minor; PTH and vitamin D are much more important.)

  • Synthesis of T3/T4:

    1. Thyroglobulin (TG) synthesized in follicular cells → secreted into follicle.

    2. Iodide actively transported into thyrocytes via Na/I symporter → sent to lumen via pendrin.

    3. Thyroid peroxidase (TPO) oxidizes iodide to iodine → attaches to TG (MIT/DIT formation).

    4. Coupling: MIT + DIT → T3, DIT + DIT → T4.

    5. TG with hormones stored in colloid.

    6. Endocytosis → breakdown → release of T3/T4 into blood.

  • Transport:

    • Mostly bound to thyroxine-binding globulin (TBG).

    • Only free hormone is active.

  • Peripheral conversion: T4 → T3 (active form) by deiodinase.

  • Regulation: Hypothalamus (TRH) → Pituitary (TSH) → Thyroid (T3/T4). Negative feedback by T3/T4.

3. Clinical significance of thyroid

  • Hypothyroidism:MORE e.g., Hashimoto thyroiditis, iodine deficiency, congenital hypothyroidism.

  • Hyperthyroidism:MORE e.g., Graves disease, toxic multinodular goiter.

  • Thyroid nodules/cancers: papillary, follicular, medullary, anaplastic.

  • Surgical risks: nerve injury, parathyroid removal → hypocalcemia.

4. Parathyroid glands

  • Number & location: Usually 4 small glands, posterior surface of thyroid.

  • Embryology:

    • Superior parathyroids → from 4th pharyngeal pouch.

    • Inferior parathyroids → from 3rd pharyngeal pouch. (Hence, their position may vary more.)

  • Blood supply: Inferior thyroid artery.

  • Cells:

    • Chief cells → secrete parathyroid hormone (PTH).

    • Oxyphil cells (function less clear).

  • PTH function:

    • Increases serum calcium, decreases phosphate.

    • Bone: stimulates osteoclasts indirectly (via osteoblasts).

    • Kidney: ↑ Ca reabsorption, ↓ phosphate reabsorption, activates vitamin D.

    • Gut: via vitamin D → ↑ Ca and phosphate absorption.

5. Clinical significance of parathyroid

  • Hyperparathyroidism → hypercalcemia, bone resorption, kidney stones.

  • Hypoparathyroidism → hypocalcemia (tetany, seizures).

  • Pseudohypoparathyroidism → resistance to PTH (low Ca, high phosphate, high PTH).

  • Surgical risk: Thyroidectomy may accidentally remove or damage parathyroid glands → hypoparathyroidism + hypocalcemia.

⚠️ Example: In DiGeorge syndrome (22q11.2 deletion) → failure of 3rd and 4th pouch development → absent parathyroids → hypocalcemia.

✅ In summary:

  • Thyroid → metabolism (T3/T4) + calcitonin.

  • Parathyroids → calcium homeostasis (PTH).

  • Both share close anatomy → surgery on one often risks the other.


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