Coupon Accepted Successfully!


Gross Anatomy

  1. The thyroid extends from the level of the fifth cervical vertebra down to the first thoracic.
  2. The gland varies from an H to a U shape and is formed by 2 elongated lateral lobes connected by a median
  3. isthmus (with an height of 12-15 mm) overlying the second to fourth tracheal rings.
  4. Each lobe is 50-60 mm long. Thyroid weight averages 25-30 g in adults.
  5. Under the middle layer of deep cervical fascia, the thyroid has an inner true capsule. Extensions of this
  6. capsule within the substance of the gland form numerous septae, which divide it into lobes and lobules. The lobules are composed of follicles. Q
  7. Epithelial cells are of 2 types: principal cells (ie, follicular) and parafollicular cells (ie, C, clear, light cells).
  8. Principal cells are responsible for formation of the colloid (iodothyroglobulin), whereas parafollicular cells
  9. produce the hormone calcitonin, a protein central to calcium homeostasis.
  10. Parafollicular cells lie adjacent to the follicles within the basal lamina
  11. Blood Supply: The thyroid gland has an abundant blood supply with normal flow rate of 5 ml/g/ min.        
    Although the thyroid represents about 0.4% of body weight it accounts for 2% of total blood flow.
    This abundant blood supply is provided from the four major thyroid arteries.
    1. The superior pair arise from the external carotid and reach the upper poles of the thyroid, where they
    2. break into a number of branches and enter the substance of the gland.
    3. The inferior pair spring from the thyrocervical trunk of the subclavian arteries and enter the lower
    4. poles from behind. Frequently, a fifth artery, the thyreoidea ima, from the arch of the aorta, enters the
    5. thyroid in the midline (3%). The branching of the large arteries takes place on the surface of the gland, where they form a network.
    6. Only after much branching are small arteries sent deep into the gland. These penetrating vessels arborize
    7. among the follicles, finally sending a follicular artery to each follicle. This, in turn, breaks up into the rich
    8. capillary network surrounding the follicle.
    9. The veins emerge from the interior of the gland and form a plexus of vessels under the capsule. These
    10. drain into the internal jugular, the brachiocephalic, and occasionally the anterior jugular veins.
  12. Innervation
    1. The gland receives fibers from both sympathetic and parasympathetic divisions of the autonomic nervous system.
    2. The sympathetic fibers are derived from the cervical ganglia and enter the gland along the blood vessels. The parasympathetic fibers are derived from the vagus and reach the gland by branches of the laryngeal nerves.
    3. Lymphatics: A rich plexus of lymph vessels is in close approximation to the individual follicles, but no   unique role in thyroid function has been assigned to this system. 
    4. The Secretory Unit - The Follicle


Description: Graphic copy


  1. The adult thyroid is composed of follicles, or acini. The cells of the follicles are the makers of hormone; the lumina are the storage depots.
  2. The average diameter is 300 microns. Under chronic TSH stimulation such as occurs with iodide deficiency, the height increases, and the term columnar is applied.
  3. Such stimulation, which increases colloid resorption, also leads to a reduction in size the follicular lumen. As a result, the height of the epithelium is often inversely proportional to the diameter of the lumen of the follicle.
  4. In addition to the acinar cells, there are individual cells or small groups of cells that are seen not to extend to the follicular lumen and which may appear as clusters between follicles.
  5. These light cells, or C-cells, are a distinct category probably derived from the neural crest via the ultimobranchial body. C-cells secrete calcitonin ( "thyrocalcitonin") in response to an increase in serum calcium. 

Test Your Skills Now!
Take a Quiz now
Reviewer Name