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Canine Thyroid Tumors—A Surgical Oncologist’s Perspective

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What’s up with fine needle aspirate of canine thyroid masses—too dangerous?

This is a question I get all the time!  I think we’ve all had a little fear put into us at one time or another about FNA of thyroid tumors.  It is actually very safe to FNA thyroid masses, and it is encouraged!  Other tumors can occur in this same location (for example: soft tissue sarcomas, mast cell tumors), and it is good to know what you’re up against.  Additionally, it would be very rare to have significant bleeding from FNA (though they can bruise).  Thyroid storm (sudden release of a large amount of thyroid hormone, causing multiorgan dysfunction) is a very rare occurrence; there is only one case described in the veterinary literature.

A bigger issue with FNA is blood contamination.  Thyroid tumors are very vascular, and if you use the pin cushion/woodpecker technique with too many redirects, the hub of the needle quickly fills with blood and can dilute your sample.  I usually only redirect two or three times to minimize this.  I will also sometimes add a syringe with a ml or two of air but no active negative or positive pressure to “stabilize” against a lot of blood filling the hub.  The good news is thyroid tumors typically easily shed cells, and therefore if you can minimize blood contamination you can usually easily get a cytologic diagnosis.

As a side note, cytology often cannot reliably differentiate between thyroid adenoma and carcinoma.  However, 90% of thyroid tumors in dogs are carcinoma, so while I don’t destroy all owner hope of benign disease, realistically owners should plan for a diagnosis of malignancy.


Should I avoid biopsy of thyroid tumors?

Yes, I strongly discourage biopsy of thyroid tumors.  This is really when the vascularity can become a problem and they can continually bleed from the biopsy site; I have seen dogs fatally bleed from a thyroid tumor biopsy site.  Additionally, since they shed cells so well on FNA, biopsy is typically “overkill” (no pun intended, ha ha) and not necessary.


Can I remove canine thyroid tumors in general practice?

What a great question, I’m glad you asked! I am a big supporter of general practitioners who are interested in expanding their surgical skills.  I will preface my next comments by saying that a proportion of thyroid carcinomas are invasive and are NOT FUN to remove surgically.  These tumors will invade into the vasculature (and send thrombi down into the chest sometimes) as well as entrap nerves and other critical structures.  However, many thyroid tumors are easily resectable (my techs will tell you I call them “neck neuters” LOL).  How can you differentiate between the “neck neuter” variety and the invasive variety?  The most ideal way is a CT scan, which would provide 3D imaging of the area.  Ultrasound is another option, and an experienced ultrasonographer can often determine invasiveness.  A less sensitive way (but one I still use!) is palpation; if a thyroid tumor wiggles medial to lateral and cranial to caudal very easily, it is very often the “neck neuter” variety.


My recommendation:

  • Offer referral and document you did so.  You don’t want to be caught in an invasive tumor situation in surgery and not have offered referral.
  • If the owner declines referral and you have CT or ultrasound, offer imaging and document you did so.
  • If the owner declines and the tumor is mobile as described above, and you are interested in doing the surgery, go for it!  As always, DISCUSS THE RISKS and DOCUMENT!

Can dogs be hyperthyroid?

Yes!  10% of thyroid tumors in dogs are functional, meaning the tumor is hypersecreting.  This is important to know pre-operatively, because following removal some dogs may require tapering supplementation while the sad remaining thyroid kicks back in.


Can thyroid tumors be bilateral?

Yep, in fact around 1/3 of thyroid tumors are bilateral.  This is important to realize if you are going to surgery on a thyroid tumor, because these dogs will likely need lifelong supplementation for thyroid and parathyroid function.  T4 and ionized calcium should be measured post-operatively in these patients.


If I am doing surgery, what structures should I be aware of?

This is not a surgery where we are trying to get wide margins.  We should be hugging, but not entering, the capsule of the thyroid/tumor. 

  • Medially to the thyroid is the recurrent laryngeal nerve and trachea.  Care should be taking to stay lateral to the recurrent laryngeal nerve.
  • Dorsally to the thyroid is the carotid artery and vagosympathathetic trunk.  Care should be taken to dissect the thyroid tumor free from these structures. 

Once the tumor/thyroid has been safely dissected from these structures, it’s only significant attachments (assuming it is not invasive) will be to the cranial and caudal artery and vein, which can be large with thyroid carcinoma and should be ligated.


What wasn’t discussed?

Staging, adjuvant treatment, and alternative treatment options are beyond the scope of this article, but these should absolutely be discussed with owners, as well!


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 About the Author: Jess Barrera, DVM, MS, DACVS-SA, ACVS Fellow Surgical Oncology

Dr. Jessica Barrera graduated vet school from Colorado State in 2006. She then did a rotating internship at Animal Surgical and Emergency Center, then a surgery internship at Oregon State University, then back to Colorado State University for small animal surgery residency as well as a one year fellowship in surgical oncology. She's currently in private specialty practice in Northern Colorado consisting of ~60% surgical oncology, ~20% minimally invasive soft tissue and orthopedics, and ~20% other soft tissue and orthopedics. Her professional passion (besides being in the operating room) is encouraging, supporting, and bringing confidence general practitioners in the realm of surgery.


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