The thyroid gland
Development
- Thyroid gland develops from the floor of embryological pharynx
- Occurs between the tuberculum impar and the copula linguae
- Diverticulum known as thyroglossal duct invaginates
- Becomes bilobed to form the thyroid lobes
- Site corresponds to junction of anterior two thirds and posterior
one third of tongue
- This is known as foramen caecum
Anatomy
Relations
- Lies below thyroid cartilage
- Carotid sheath is posterior and lateral
- Parathyroids lie posterior
- Recurrent laryngeal nerve and external laryngeal nerve lie medially
- Recurrent laryngeal nerve is closely related to the inferior thyroid
artery
- The two lobes connected at an isthmus
Arterial supply
- Superior thyroid artery is a branch of the external carotid
- Inferior thyroid artery is a branch of the thyrocervical trunk
- The thyrocervical artery is a branch the subclavian artery
Venous drainage
- Superior thyroid vein drains into the internal jugular vein
- Middle thyroid vein drains into the internal jugular vein
- Inferior thyroid vein drains into the brachiocephalic
Lymph drainage
Histology
- The thyroid gland is composed of follicles that selectively absorb
iodine and concentrate it to produce thyroid hormones
- Twenty-five percent of all the body iodine is in the thyroid gland
- The follicles are made of a single layer of thyroid epithelial cells
- These secrete T3 and T4
- Inside the follicles is a colloid which is rich in a thyroglobulin
- It serves as a reservoir of materials for thyroid hormone production
- The spaces between the thyroid follicle spheres are filled with the
other type of thyroid cells, parafollicular cells or C cells
- These secrete calcitonin
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