ANA 301 Thyroid And Parathyroid Gland

Question 1

GENERAL INTRODUCTION to thyroid and parathyroid

Answer 1

•They are both endocrine glands positioned anteriorly in the neck.
•Both glands begin as pharyngeal outgrowths that migrate caudally to their final position as development continues.
•The thyroid gland is a large, unpaired gland, while the parathyroid glands, usually four in number, are small and are on the posterior surface of the thyroid gland.

Question 2

Thyroid gland and hormone and target organ
Hormone of parathyroid and target organ

Answer 2

•located anteriorly in the neck at the level of the C5-T1 vertebrae
• Consists primarily of right and left lobes, anterolateral to the larynx and trachea.
• A relatively thin isthmus unites the lobes over the trachea, usually anterior to the second and third tracheal rings.
•It produces thyroid hormone, which controls the rate of metabolism, and calcitonin, a hormone controlling calcium metabolism.

•The thyroid gland affects all areas of the body except itself and the spleen, testes, and uterus.
•The hormone produced by the parathyroid glands, parathormone (PTH), controls the metabolism of phosphorus and calcium in the blood.
•The parathyroid glands target the skeleton, kidneys, and intestine

Question 3

Arteries of the Thyroid Gland

Answer 3

The highly vascular thyroid gland is supplied by
•superior thyroid arteries (1st branch of the external carotid) supply the superior pole

• inferior thyroid arteries (the largest branches of the thyrocervical trunks arising from the subclavian arteries) provide the primary blood supply to the posterior aspect of the thyroid gland and also supply the inferior poles

•The right and left superior and inferior thyroid arteries anastomose extensively within the gland, ensuring its supply while providing potential collateral circulation between the subclavian and external carotid arteries

Question 4

Veins of the Thyroid Gland

Answer 4

• superior thyroid veins : drain the superior poles
•the middle thyroid veins: drain the middle of the lobes
•Inferior thyroid veins: drain the inferior poles
•The superior and middle thyroid veins drain into the IJVs;
•the inferior thyroid veins drain into the brachiocephalic veins posterior to the manubrium

Question 5

Lymphatic Drainage of the Thyroid Gland

Answer 5

• vessels which pass initially to prelaryngeal drains in turn to the superior deep cervical nodes
•Vessels which pass from the pretracheal and paratracheal nodes drains inturn into the inferior deep cervical lymph node
• Some lymphatic vessels may drain into the brachiocephalic lymph nodes or the thoracic duct

Question 6

Nerves of the Thyroid Gland

Answer 6

The nerves of the thyroid gland are derived from the:
• superior, middle, and inferior cervical sympathetic ganglia
•They cause constriction of blood vessels
• Endocrine secretion from the thyroid gland is hormonally regulated by the pituitary gland

Question 7

Thyroglossal duct cysts

Answer 7

•Development of the thyroid gland begins in the floor of the embryonic pharynx at the site indicated by a small pit, the foramen cecum, in the dorsum of the postnatal tongue. Subsequently, the developing gland descends from the tongue into the neck, passing anterior to the hyoid and thyroid cartilages to reach its final position anterolateral to the superior part of the trachea.

During this descent, the thyroid gland is attached to the foramen cecum by the thyroglossal duct
•This duct normally disappears but remnants of epithelium may remain and form a thyroglossal duct cyst at any point along the path of its descent

Question 8

Enlargement of the Thyroid Gland

Answer 8

•A non-neoplastic and non-inflammatory enlargement of the thyroid gland, other than the variable enlargement that may occur during menstruation and pregnancy, is called a goiter.
• A goiter results from a lack of iodine.
• It is common in certain parts of the world where the soil and water are deficient in iodine.
•The enlarged gland causes a swelling in the neck that may compress the trachea, esophagus, and recurrent laryngeal nerves

Question 9

Pyramidal Lobe of the Thyroid Gland

Answer 9

•Approximately 50% of thyroid glands have a pyramidal lobe.
• This lobe, which varies in size, extends superiorly from the isthmus of the thyroid gland, usually to the left of the median plane; the isthmus may be incomplete
•A band of connective tissue, often containing accessory thyroid tissue, may continue from the apex of the pyramidal lobe to the hyoid

Question 10

Thyroidectomy

Answer 10

•Excision of a malignant tumor of the thyroid gland or other surgical procedure sometimes necessitates removal of the gland (thyroidectomy).
• In the surgical treatment of hyperthyroidism, the posterior part of each lobe of the enlarged thyroid is usually preserved, a procedure called near-total thyroidectomy, to protect the recurrent and superior laryngeal nerves and to spare the parathyroid glands.
•Postoperative hemorrhage after thyroid gland surgery may compress the trachea, making breathing difficult. The blood collects within the fibrous capsule of the gland

Question 11

Thyroid Ima Artery

Answer 11

•In approximately 10% of people, a small, unpaired thyroid ima artery arises from the brachiocephalic trunk
• however, it may arise from the arch of the aorta or from the right common carotid, subclavian, or internal thoracic arteries.
•This small artery ascends on the anterior surface of the trachea, which it supplies, and continues to the isthmus of the thyroid gland, where it divides and supplies it.
•The possible presence of this artery must be considered when performing procedures in the midline of the neck inferior to the isthmus because it is a potential source of bleeding

Question 12

Parathyroid Glands

Answer 12

•The small flattened, oval parathyroid glands usually lie external to the thyroid capsule on the medial half of the posterior surface of each lobe of the thyroid gland, inside its sheath
We have the:
•superior parathyroid glands
•Inferior parathyroid glands
•The superior parathyroid glands usually lie slightly more than 1 cm superior to the point of entry of the inferior thyroid arteries into the thyroid gland.
•The inferior parathyroid glands usually lie slightly more than 1 cm inferior to the arterial entry point
• Most people have four parathyroid glands.
•Approximately 5% of people have more; some have only two glands.

• The superior parathyroid glands, more constant in position than the inferior ones, are usually at the level of the inferior border of the cricoid cartilage.
• The inferior parathyroid glands are usually near the inferior poles of the thyroid gland, but they may lie in various positions
•In 1-5% of people, an inferior parathyroid gland is deep in the superior mediastinum

Question 13

Arterial supply to the Parathyroid Glands

Answer 13

Arterial supply to the Parathyroid Glands
Because the inferior thyroid arteries provide the primary blood supply to the posterior aspect of the thyroid gland where the parathyroid glands are located, branches of these arteries usually supply the parathyroid glands

However, they may also be supplied by branches from :
•superior thyroid arteries
•the thyroid ima artery;
•or the laryngeal, tracheal, and esophageal arteries.

Question 14

Parathyroid veins

Answer 14

drain into the thyroid plexus of veins of the thyroid gland and trachea

Question 15

Lymphatic drainage

Answer 15

Lymphatic vessels from the parathyroid glands drain with those from the thyroid gland into
deep cervical lymph nodes and
paratracheal lymph nodes

Question 16

Inadvertent Removal of the Parathyroid Glands

Answer 16

The variable position of the parathyroid glands, especially the inferior ones, puts them in danger of being damaged or removed during surgical procedures in the neck.
The superior parathyroid glands may be as far superior as the thyroid cartilage,
and the inferior ones may be as far inferior as the superior mediastinum.
The aberrant sites of these glands are of concern when searching for abnormal parathyroid glands, as may be necessary in treating parathyroid adenoma, an ordinarily benign tumor of epithelial tissue associated with hyperparathyroidism.
Atrophy or inadvertent surgical removal of all the parathyroid glands results in tetany, a severe neurologic syndrome characterized by muscle twitches and cramps
To safeguard these glands during thyroidectomy, surgeons usually preserve the posterior part of the lobes of the thyroid gland.
In cases in which it is necessary to remove all of the thyroid gland (e.g., because of malignant disease), the parathyroid glands are carefully isolated with their blood vessels intact before removal of the thyroid gland.
Parathyroid tissue may also be transplanted, usually to the arm so it will not be damaged by subsequent surgery or radiation therapy.