Chapter 121: Thyroid and Parathyroid Glands

Question 1

Where are the thyroid glands located?

Answer 1

Generally: lateral/ventral to trachea spanning first 5-8 rings

Cranial pole of the right is found at the caudal border of the cricoid cartilage
Left gland is more caudal extending along the 3rd to 8th tracheal rings

Question 2

What is the bridge of tissue connecting the left and right thyroid glands called?

Answer 2

isthmus glandularis

Question 3

What is the normal size of a canine thyroid gland?

Answer 3

5cm x 1.5cm x 0.5cm medium dog

Question 4

What structures lie near the right thyroid gland?

Answer 4

Carotid sheath (common carotid, internal jugular, vagosympathetic trunk)
Tracheal duct
Esophagus (dorsolaterally)
Recurrent laryngeal nerves (medially)

**Esophagus displaces the carotid sheath on left so no contact

Question 5

The cranial thyroid artery arises from which artery?

Answer 5

The common carotid artery - it is the first branch off the common carotid.

Question 6

The caudal thyroid artery arises from which artery?

Answer 6

the brachiocephalic artery

Most cats do NOT have a caudal thyroid artery *****

Question 7

What nerve innervates the thyroid?

Answer 7

thyroid n. from the cranial laryngeal n. off the vagus n.

Question 8

How are thyroid hormones produced?

Answer 8

Thyroglobulin produced within the thyroid and stored within the thyroid follicle (iodine is necessary)
- In follicular cells thyroglobulin is hydrolyzed into thyroxine (T4) and triiodothyronine (T3)

Question 9

Which hormone (T3 or T4) is the major hormone in terms of biologic activity?

Answer 9

T3

Question 10

What is unique about the thyroid blood supply of the cat?

Answer 10

In most cats, the caudal thyroid artery is absent. IMPORTANT

Question 11

Into what ducts does the lymph of the thyroids eventually drain?

Answer 11

Right: Right lymphatic duct
Left: Tracheal duct

Question 12

Ectopic thyroid tissue is common in dogs and cats - where can it be found?

Answer 12

Anywhere from base of tongue to base of heart. IMPORTANT

Question 13

What % of T3 and T4 circulate unbound to protein?

Answer 13

Less than 1%

Question 14

What substances regulate thyroid hormone synthesis?

Answer 14

• Thyrotropin (TSH) from the pituitary gland
• Thyrotropic-releasing hormone (TRH) from the hypothalamus (which stimulates TSH release).

Question 15

Where is PTH made?

Answer 15

PTH is synthesized, stored and secreted by the chief cells of the parathyroid gland

Question 16

What are the main effects of PTH?

Answer 16

Increase Calcium in blood / Decrease phosphorus

Bone: Ca and phosphate resorption
Kidneys: Rapid decrease in excretion of Ca and increase in excretion of phosphorus.

Intestines: Indirect effect -> PTH makes kidneys increase formation of calcitriol from Vitamin D. That calcitriol increases absorption of Ca and phosphorus in the GI

Question 17

Other than PTH, what other hormone is involved with Ca homeostasis? How does it work?

Answer 17

Calcitonin (produced by thyroid gland parafollicular cells, aka C-cells)

• Prevents postprandial hyperCa by decreasing bone resorption but has no effect on kidneys or intestines

Question 18

What imaging method is most useful for diagnosis and anatomical localization of hyperfunctioning thyroid tissue?

Answer 18

Scintigraphy
- Technetium 99m - pertechnetate (99mTcO4)

• Pertechnetate normally concentrates in thryoid, salivary and gastric mucosa

Question 19

What is the main goal of pre-op treatment of hyperthyroid cats?

Answer 19

Acheive euthyroid in order to check for azotemia - if azotemic, do not recommend irreversible treatments (stay on methimazole)

Question 20

What muscles must be separated on the approach to the thyroid?

Answer 20

• Sternohyoid
• Sternothyroid

Question 21

What is the blood supply of the thyroid?

Answer 21

Cranial and caudal thyroid arteries.

Question 22

What is the venous drainage of the thyroid?

Answer 22

Cranial and caudal thyroid veins -> internal jugular vein

Question 23

What lymph nodes drain the thyroid?

Answer 23

caudal deep cervical LN. If absent, go to medial retropharyngeal and ultimately to the ducts.

Question 24

Ectopic parathyroid is uncommon in dogs, but can be found in what % of cats? What is it associated with?

Answer 24

35-50% of cats, associated with thymus.

Question 25

What % of thyroid hormone is bound to protein in plasma?

Answer 25

99%
It is the free hormone that is active/enters cells.

Question 26

The active form of calcium Ca+ (ionized) is tightly regulated by PTH. How does Ca+ concentration affect PTH?

Answer 26

Negative feedback - increased Ca+ decreases PTH secretion.

PTH has a very short half life and rapid effect.

Question 27

Thyroid gland parafollicular cells (C-cells) produce what?

Parathyroid chief cells produce what?

Answer 27

Thyroid: Calcitonin

Chief cells: PTH

Question 28

True or false? T3 is the major secretory product of the thyroid gland.

Answer 28

False - T4 is major secretion but T3 is active. 40-60% of T3 is derived from monodeiodination (loss of an iodide) in peripheral tissues.

Question 29

True or false? Only free hormones (the 1%) regulate the negative feedback loop for TSH.

Answer 29

True, and primarily T3

Question 30

Where are the parathyroids usually found (though location is variable)?

Answer 30

Externals are at the cranial dorsolateral margin of the thyroid.

Internals are at the caudal pole of the thyroid.

Question 31

True or false: Vascular supply, innervation, and venous/lymph drainage to the parathyroids are similar to the thyroids?

Answer 31

True. The externals get supply from cranial thyroid. The internals get theirs from thyroid parenchyma.