Thyroid anatomy

Question 1

Where is the thyroid gland located?

Answer 1

Is located around the cranial part of the trachea

Question 2

What are the two types of secretory cell in the thyroid?

Answer 2

Follicular cells and parafollicular cells (C-cells)

Question 3

What do the follicular cells produce?

Answer 3

Thyroid hormone: T3 and T4

Question 4

What do the parafollicular cells produce?

Answer 4

Calcitonin which lowers blood calcium.

Question 5

What tissues surround the thyroid?

Answer 5

The thyroid is surrounded medially by the trachea, thyroid artery and recurrent laryngeal nerve.
Ventrolaterally there is the sternohyoid, sternothyroid muscle and veins.
Dorsally there is the oesophagus, parathyroids, carotid sheath and contents.

Question 6

Which species have a functional isthmus?

Answer 6

Pigs, humans, cattle

Question 7

Which species have a fibrous isthmus/ separate glands?

Answer 7

Dogs, cats, small ruminants, horses – separate L & R glands

Question 8

How are follicular cells formed?

Answer 8

Follicular cells are a down-growth from pharyngeal endoderm of developing tongue at site of future foramen caecum.

Question 9

How are parafollicular cells formed?

Answer 9

Parafollicular or C-cells from ultimobranchial body the endoderm of most caudal pharyngeal pouch. (But may be of neural crest origin?). They develop from the 4th

Question 10

What is the blood supply to the thyroid?

Answer 10

Cranial thyroid a. (from common carotid a.)

Caudal thyroid a. (from brachiocephalic trunk)

Question 11

What is the venous drainage of the thyroid?

Answer 11

Thyroid vv. (drain variably to internal and external jugular vv. and brachiocephalic v.)

Question 12

What is the innervation of the thyroid?

Answer 12

Has a complete autonomic supply that is vasomotor only.
Sympathetic fibres come from the cranial cervical ganglion.
Parasympathetic fibres leave the vagus at the distal vagal ganglion.
The S & PS then axons travel together in the cranial laryngeal nerve.

Question 13

What secretory cells do the parathyroids contain?

Answer 13

Chief or principal cells

Question 14

What do the parathyroids secrete?

Answer 14

Parathyroid hormone -> raises blood calcium.

Question 15

Where is parathyroid III in cats and dogs?

Answer 15

Cranial (dogs) or caudal (cats) lobes of thyroid, often embedded under the capsule.
it sits in a doughnut of fat

Question 16

Where is parathyroid four in cats and dogs?

Answer 16

Deep within thyroid

Question 17

Where are the parathyroids in ruminants?

Answer 17

external parathyroid (III) close to the bifurcation of the common carotid artery.
internal parathyroid (IV) associated with thyroid gland.

Question 18

Where are the parathyroids in horses?

Answer 18

external (lower) parathyroid III usually near the trachea ~15cm cranial of the thoracic aperture, but very variable.
internal (upper) parathyroid IV around cranial part of thyroid, or inside