The Thyroid
Definition
o Is an important endocrine gland which secretes the hormone thyroxine
Situation
o Situated in the front of neck.
o In front of the larynx and trachea
o At the level of 5th, 6th and 7th cervical vertebrae
Structure
o Highly vascular
o Fibrous capsule
o Two lobes right and left
o One on either side of the trachea
o Narrow isthmus connecting the two lobes
Histology
o The gland consists of cubical epithelial cells
o The epithelial cells form spherical follicles
o Follicles contain a protein called colloid
o The colloid combines with the hormone thyroxine and stores the hormone - releases it when needed.
o Also the gland has parafollicular cells called C - cells calcitonin
Blood supply
o superior thyroid artery from external carotid
o inferior thyroid artery from the thyrocervical trunk of the subclavian artery
o both the arteries are paired ( i.e. right and left)
o venous return into the internal jugular veins
o two parathyroid glands lie against the posterior surface of each lobe
Relations
o Anteriorly : fascia, flat muscles of the neck and skin
o Laterally: carotid sheath
o Medially: trachea, oesophagus and tracheo-oesophageal groove and recurrent laryngeal nerves
o Posteriorly: parathyroids
Hormones produced and their clinical significance
o Thyroxine
o triiodothyronine, tetraiodothyronine (T3 and T4)
o Deficiency causes hypothyroidism
o Excess causes hyperthyroidism thyrotoxicosis which can be primary or secondary
o Enlargement of the gland causes a swelling in front of the neck called goitre.
