Abstract
Dear Editors, – I would like to commend Dr Carney and her group for the time and effort expended that resulted in these very important Guidelines 1 for a potentially devastating, but curable, feline disease.
I have performed thyroidectomies since the late 1980s and published a technique I called the parathyroid transplant approach in 1995. 2 During the 1990s it was not uncommon for me to perform up to three of these surgeries on any given day. In 2004 my practice was granted a license by the State of Texas to perform radioiodine therapy. My associates and I have treated over 500 cats in the past 12 years. Thus, I have extensive experience in both modalities as well as of treating hundreds of cats with methimazole. I tell you this so you can understand the depth of my experience and the breadth of my approach to this disease.
Although I currently only perform five or fewer thyroidectomies per year, there is one situation in which my associates, Drs Lisa Restine and Anne Romeo, and I have found it advantageous to combine methimazole therapy, surgery and radioiodine therapy. We call this the hybrid approach. I do not see it addressed as such in the Guidelines.
The situation is a cat with marked enlargement (5/6 or 6/6 on my palpation scale 3 ) of both thyroid lobes and a total thyroxine level (TT4) of 20 mg/dl or more. This cat typically requires a dose of ⩾6 mCi of radioiodine to destroy the tumor, but that dose often destroys the normal thyroid tissue, resulting in permanent hypothyroidism. Therefore, I will prescribe methimazole for 2–6 weeks to achieve euthyroidism, remove one thyroid lobe surgically, recheck the TT4 2–3 weeks later, and then administer a dose of radioiodine, based on the postsurgical TT4. This dose will be much lower than I would have used without surgery and should destroy the thyroid tumor while sparing normal thyroid tissue.
Although the cost is greater with this approach, the outcome should be a cat with no thyroid tumor and normal thyroid function, avoiding the need for long-term thyroid hormone supplementation. In addition, this approach permits a histopathological examination of the largest thyroid lobe, making determination of adenocarcinoma, if present, more likely. If it is present, the subsequent dose of radioiodine can be appropriately determined.
There are a couple of points relating to surgery that one should be aware of when using the hybrid approach. Firstly, when a thyroid lobe is very large, its external parathyroid gland is usually overgrown by the rapidly expanding thyroid adenoma, sometimes making it impossible to find during surgery. However, removal of the internal and external parathyroid glands of one thyroid lobe will not result in hypocalcemia as long as there is at least one parathyroid gland present and functioning on the contralateral side. Since it is not essential that the external parathyroid gland be located and its blood supply preserved, the surgery is actually even easier than normal. Secondly, in a cat with a very large thyroid lobe, it is not uncommon for the recurrent laryngeal nerve to be adhered to the thyroid lobe, requiring very careful (but doable) dissection. Failure to achieve this will result in laryngeal paralysis.