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Case Report: Thyroid Cystadenoma in a Cat

A 13 year old male, neutered DLH cat was referred for assessment of a ventral cervical swelling.  The mass had first been noted by the referring vet two months previously when fine-needle aspirates yielded serous fluid with haemorrhage, but no abnormal cells. The mass had subsequently reappeared and the cat was then referred. Thyroxine levels had been found to be within normal limits. 

fig.1 - dorsal T2W slice of mid-cervical region, showing large, expansile fluid-filled mass compressing the surrounding soft tissues. There is no evidence of infiltrative growth into neighbouring structures.

On presentation, the cat was bright, alert and comfortable.  General physical examination revealed a good body condition.  There was a firm, mobile, non-painful swelling over the region of the right jugular groove cranial to the manubrium.  Routine haematology and biochemistry (including repeat T4) were unremarkable.  Thoracic radiology revealed no evidence of metastatic disease.  MR-scans of the neck revealed a large cystic structure within the soft tissues, located immediately caudal to the larynx.  There was a small soft tissue mass indenting the wall of the cyst medially, thought to be a remnant of the right thyroid gland (fig. 1).   Although the mass was displacing the structures of the neck medially, there was no evidence of invasion into the surrounding soft-tissues.

 The cyst was approached via a ventral, midline, cervical incision – as for a standard thyroidectomy.  An extracapsular resection was performed and the cyst along with attached glandular material (assumed to be the parathyroid and remainder of the right thyroid gland) were excised (fig. 2).  All tissue was submitted for histopathology.  The cyst contents were also resubmitted for cytology and bacteriology.  A passive wound drain was placed to exit close to the skin incision, which was then closed in a routine manner. The cat made an uneventful recovery from anaesthesia and the wound-drain was removed the following day, before the cat was discharged. 

 Cytology revealed cellular debris and bacteriology yielded no growth.  Histopathology confirmed the mass to be a thyroid cyst-adenoma, giving a favourable long-term prognosis.

 Discussion:

fig. 2 - Intraoperative view of cyst, showing attached glandular tissue.

Cystic thyroid disease has been reported in the cat as both functional1 i.e. causing hyperthyroidism and non functional2. In one article a series of four cases were reviewed. One was a thyroid cyst, one a parathyroid cyst and two were thyroid cystadenomas2

Differential diagnoses for this condition would also include branchial retention cyst or dermoid cyst3. Thyroid cysts appear to have good prognosis following surgical removal2 although there is a report of one cat dying subsequent to radioactive iodine treatment, presumably of aspiration pneumonia1.  Extracapsular thyroidectomy is not usually recommended due to the risk of hypocalcaemia following this procedure, although this is very rare when performed unilaterally. The reason it was chosen in this case was to minimise the risk of leaving malignant cells behind, given the open diagnosis at time of surgery.

Imaging of thyroid cysts has been described using ultrasound2 predominantly, although there are reports in the literature relating to imaging thyroid glands using MRI in dogs4 and cats1. In the human field both CT and MRI are used for the assessment of goitre and MRI is thought to be superior to CT in detecting early oesophageal and tracheal invasion as well as involvement of the recurrent laryngeal  nerve4. In the case reported here, MRI was particularly useful in identifying the mass as being associated with the right thyroid gland, as well as assessing for any involvement of surrounding structures.

References:

1. Hofmeister E., Kippenes H., Mealey K.L., Cantor G.H. Lohr C.V. Functional cystic thyroid adenoma in a cat. J Am Vet Med Assoc. 2001 Jul 15; 219(2):190-3

2. Phillips D.E., Radlinsky M.G., Fischer J.R. and Biller D.S. Cystic thyroid and parathyroid lesions in cats. J Am Anim Hosp Ass 39:349-354 (2003)

3. Tolbert K., Brown H.M., Rakich P.M., Radlinsky M.A.G., Ward C.R. Dermoid cysts presenting as enlarged thyroid glands in a cat. J Feline Med Surg. August 2009;11(8):717-9

4. Taeymans O., Peremans K., Saunders J.H. Thyroid imaging in the dog: current status and future directions. J Vet Intern Med 2007; 21:673-684