thyroid gland anatomy and physiology best and easy explained..

1. Anatomy of the thyroid gland

2. Topic outline
• Definition:
• Description:Location,Structure,Lobes, Relationships with
neighbouring structures,vascular anatomy,lymphatics of thyroid n
parathyroid glands
• Clinical notes:Hypo n hyper thyroidism,hyperparathyroidism

3. Background
• The thyroid gland weighs 10 to 20 grams in normal adults. Thyroid
volume measured by ultrasonography (US) is slightly greater in men
than women
• it increases with age and body weight.
• it decreases with increasing iodine intake.
• The thyroid is one of the most vascular organs in the body.

4. …
• The normal thyroid gland is immediately caudal to the larynx and
encircles the anterolateral portion of the trachea. The thyroid is
bordered by the trachea and esophagus medially and the carotid
sheath laterally. The sternocleidomastoid muscle and the three strap
muscles (sternohyoid, sternothyroid, and the superior belly of the
omohyoid) border the thyroid gland anteriorly and laterally

5. …
• It consists of two lobes connected by a narrow isthmus. Each lobe is
pyramidal in shape, with its apex directed upward and its base
directed downward. The isthmus is the narrow part of the gland
connecting the two lobes. A small pyramidal lobe projecting
upward from the isthmus is often present to the left of the midline.

6. .

7. Transverse view: relationship to other
NB structures in neck

8. Histology
• Under middle layer of deep cervical fascia (pretracheal)  thyroid
inner true capsule  thin and closely adherent to the gland
• capsule extensions within the gland form septae, dividing it into
lobes and lobules
• lobules are composed of follicles = structural units of the gland 
layer epithelium enclosing a colloid-filled cavity
• colloid (pink on H&E stain) contains an iodinated glycoprotein,
iodothyroglobulin (precursor of thyroid hormones).

9. • Follicles = variable size
• surrounded by dense plexuses of fenestrated capillaries, lymphatic
vessels, and sympathetic nerves.

10. …
Epithelial cells = 2 types:
• principal (ie: follicular) – formation of colloid (iodothyroglobulin)
• parafollicular (ie: C cells -clear, light), lie adjacent to follicles w/in
basal lamina  produce calcitonin

11. Arterial supply
• The arterial blood supply to the thyroid gland is primarily from the
right and left superior and inferior thyroid arteries, derived from the
external carotid arteries and thyrocervical trunk, respectively. The
venous drainage consists of the superior, middle, and inferior thyroid
veins that drain into the internal jugular vein and innominate vein
• Superior thyroid artery is the first branch off the external carotid
artery. It extends inferiorly to the superior pole of the thyroid lobe.
In addition to supplying the thyroid, the superior thyroid artery is the
primary blood supply to approximately 15 percent of superior
parathyroid glands.
• The superior thyroid artery is a landmark for identification of the
superior laryngeal nerve, which courses with the artery until
approximately 1 cm from the superior thyroid pole

12. …
• Inferior thyroid artery is a branch of the thyrocervical trunk which
arises from the subclavian artery.
• The inferior thyroid artery courses posterior to the carotid artery to
enter the lateral thyroid. The point of entry can extend from superior
to inferior thyroid poles.
• The inferior thyroid artery also supplies the inferior parathyroid
glands and approximately 85 percent of superior parathyroid glands..
• Thyroidea ima artery is found in approximately 3 percent of
individuals and arises from the aortic arch or innominate artery and
courses to the inferior portion of the isthmus or inferior thyroid
poles.
• Surgical control of the thyroidea ima artery is essential during
thyroidectomy. The thyroidea ima artery can be quite enlarged in
patients with thyroid disease such as goiter or hyperthyroidism

16. Venous drainage
• Venous drainage Superior thyroid vein: It arises from the upper part
of the lobe. It ends into the internal jugular vein.
• Middle thyroid vein: It arises from the middle of the lobe. It ends into
the internal jugular vein.
• Inferior thyroid veins: Arise from the isthmus and lower parts of the
lobes. Descend in front of the trachea. End into the left
brachiocephalic vein.

18. Lymphatic drainage
• The lymphatic vessels of the thyroid gland drain into:
• 1) Pretracheal lymph nodes.
• 2) Paratracheal lymph nodes. - The efferent of these nodes drain into
the deep cervical lymph nodes.

20. Recurrent laryngeal nerve
• Recall: innervates all larynx except cricothyroid
• Closely assoc with ITA (see next slides for details)
• NB: ‘non recurrent LN’ ~5/1000 pt’s on R side
• When retroesophageal R SCA (right subclavian artery)
from dorsal aortic arch
• NRLN - branches from X at ~ cricoid cartilage
• directly enters the larynx without looping around SC
• L sided - only when R aortic arch and ligamentum
arteriosum concurrent w/ L retroesophageal subclavian
artery.

21. …
• The left and right recurrent laryngeal nerves lie in close proximity to
the thyroid gland and care must be taken not to damage them during
thyroid surgery.
• They branch from their respective vagus nerve within the chest and
hook around the right subclavian artery (right RL nerve), or the arch
of aorta (left RL nerve).
• The recurrent laryngeal nerve then travels back up the neck, running
between the trachea and oesophagus in the tracheoesophageal
groove. It then passes underneath the thyroid gland to innervate the
larynx.

22. …
• The RLN may course anterior or posterior to the inferior thyroid
artery. In some cases, the RLN may branch into both an anterior and
posterior position
• Vocal cord paresis or paralysis due to iatrogenic injury of the
recurrent laryngeal nerve (RLNI) is one of the main problems in
thyroid surgery. Although many procedures have been introduced to
prevent the nerve injury, still the incidence of recurrent laryngeal
nerve palsy varies between 1.5-14%.
• Injury to the recurrent laryngeal nerve has the potential to
cause unilateral vocal cord paralysis. Patients with this typically
complain of new-onset hoarseness, changes in vocal pitch, or noisy
breathing.

24. .
•Thyroid Pathology
• Hyperthyroidism, commonest is Grave’s disease
(autoimmune)
• Hypothyroidism
• In childhood leads to cretinism
• Endemic goiter from insufficient iodine in diet
• Adult hypothyroidism (myxedema): autoimmune

25. 25
Exophthalmos of
Grave’s disease
Enlarged thyroid
(goiter) from
iodine deficiency

26. Parathyroid gland
• Normal parathyroid glands are approximately the size of a grain of
rice or a lentil. Normal glands are usually about 5 by 4 by 2
millimeters in size and weigh 35 to 50 milligrams. Enlarged
parathyroid glands can be 50 milligrams to 20 grams in weight, most
typically weighing about 1 gram and 1 centimeter in size
Normal superior parathyroid glands are usually located on the posterior-
lateral surface of the middle to superior thyroid lobe. They lie under
the thyroid superficial fascia, posterior to the recurrent laryngeal nerve
and can be visualized by carefully dissecting the thyroid capsule in this
region.

27. …
• The two inferior parathyroid glands reside in the anterior mediastinal
compartment, anterior to the recurrent laryngeal nerve. They are
most often found in the thyrothymic tract, or just inside the thyroid
capsule on the inferior portion of the thyroid lobes
• The superior parathyroid glands receive most of their blood supply
from the inferior thyroid artery and also are supplied by branches of
the superior thyroid artery in 15 to 20 percent of patients. The
inferior parathyroid glands receive their end-arterial blood supply
from the inferior thyroid artery.

29. PTH function

30. Hyperparathyroidism
• Primary Hyperparathyroidism
• Normal feedback of Ca disturbed, causing increased production of PTH
• Secondary Hyperparathyroidism
• Defect in mineral homeostasis leading to a compensatory increase in
parathyroid gland function
• Tertiary Hyperparathyroidism
• After prolonged compensatory stimulation, hyperplastic gland develops
autonomous function

32. Thank you!