Chapter 121: Thyroid and Parathyroid Glands Flashcards
Where are the thyroid glands located?
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
What is the bridge of tissue connecting the left and right thyroid glands called?
isthmus glandularis
What is the normal size of a canine thyroid gland?
5cm x 1.5cm x 0.5cm medium dog
What structures lie near the right thyroid gland?
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
The cranial thyroid artery arises from which artery?
The common carotid artery - it is the first branch off the common carotid.
The caudal thyroid artery arises from which artery?
the brachiocephalic artery
Most cats do NOT have a caudal thyroid artery *****
What nerve innervates the thyroid?
thyroid n. from the cranial laryngeal n. off the vagus n.
How are thyroid hormones produced?
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)
Which hormone (T3 or T4) is the major hormone in terms of biologic activity?
T3
What is unique about the thyroid blood supply of the cat?
In most cats, the caudal thyroid artery is absent. IMPORTANT
Into what ducts does the lymph of the thyroids eventually drain?
Right: Right lymphatic duct
Left: Tracheal duct
Ectopic thyroid tissue is common in dogs and cats - where can it be found?
Anywhere from base of tongue to base of heart. IMPORTANT
What % of T3 and T4 circulate unbound to protein?
Less than 1%
What substances regulate thyroid hormone synthesis?
- Thyrotropin (TSH) from the pituitary gland
- Thyrotropic-releasing hormone (TRH) from the hypothalamus (which stimulates TSH release).
Where is PTH made?
PTH is synthesized, stored and secreted by the chief cells of the parathyroid gland
What are the main effects of PTH?
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
Other than PTH, what other hormone is involved with Ca homeostasis? How does it work?
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
What imaging method is most useful for diagnosis and anatomical localization of hyperfunctioning thyroid tissue?
Scintigraphy
- Technetium 99m - pertechnetate (99mTcO4)
- Pertechnetate normally concentrates in thryoid, salivary and gastric mucosa
What is the main goal of pre-op treatment of hyperthyroid cats?
Acheive euthyroid in order to check for azotemia - if azotemic, do not recommend irreversible treatments (stay on methimazole)
What muscles must be separated on the approach to the thyroid?
- Sternohyoid
- Sternothyroid
What is the blood supply of the thyroid?
Cranial and caudal thyroid arteries.
What is the venous drainage of the thyroid?
Cranial and caudal thyroid veins -> internal jugular vein
What lymph nodes drain the thyroid?
caudal deep cervical LN. If absent, go to medial retropharyngeal and ultimately to the ducts.
Ectopic parathyroid is uncommon in dogs, but can be found in what % of cats? What is it associated with?
35-50% of cats, associated with thymus.
What % of thyroid hormone is bound to protein in plasma?
99%
It is the free hormone that is active/enters cells.
The active form of calcium Ca+ (ionized) is tightly regulated by PTH. How does Ca+ concentration affect PTH?
Negative feedback - increased Ca+ decreases PTH secretion.
PTH has a very short half life and rapid effect.
Thyroid gland parafollicular cells (C-cells) produce what?
Parathyroid chief cells produce what?
Thyroid: Calcitonin
Chief cells: PTH
True or false? T3 is the major secretory product of the thyroid gland.
False - T4 is major secretion but T3 is active. 40-60% of T3 is derived from monodeiodination (loss of an iodide) in peripheral tissues.
True or false? Only free hormones (the 1%) regulate the negative feedback loop for TSH.
True, and primarily T3
Where are the parathyroids usually found (though location is variable)?
Externals are at the cranial dorsolateral margin of the thyroid.
Internals are at the caudal pole of the thyroid.
True or false: Vascular supply, innervation, and venous/lymph drainage to the parathyroids are similar to the thyroids?
True. The externals get supply from cranial thyroid. The internals get theirs from thyroid parenchyma.
