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Changes in thyroid pathology in hyperthyroid cats with disease duration

Peterson ME, Broome MR, Rishniw M. Prevalence and degree of thyroid pathology in hyperthyroid cats increases with disease duration: a cross-sectional analysis of 2096 cats referred for radioiodine therapy. J Feline Med Surg. 2016 Feb;18(2):92-103.

Hyperthyroidism is a commonly encountered disease of older cats (though it has been reported in animals less than one year of age). Though the underlying cause of this disease is unknown, it is characterized by a nodular growth of the thyroid gland then secretes and abundance of thyroid hormone (thyroxine or T4). This increase in thyroid hormone causes an increased metabolic rate leading to weight loss despite a ravenous appetite, increased heart rate and blood pressure, and liver, heart and kidney damage. Untreated, hyperthyroidism is eventually fatal. Traditionally the hormone secretion has been considered to be due to an adenoma, or benign cancer.

Several options exist to treat hyperthyroidism. A drug called methimazole is able to suppress the production of thyroid hormone and may be given orally or transdermally daily. A prescription diet is available that limits iodine content (a key part of T4 synthesis) and prevents thyroid hormone production. In the past, surgical thyroidectomy was the curative treatment of choice to cure hyperthyroidism; however there are several drawbacks to this procedure. The modern gold standard for cure of hyperthyroidism is radioiodine (I131) therapy. This is a fairly simple procedure consisting of injection of a radioactive substance that destroys overgrown thyroid tissue.

The current study reviewed information on 2096 cats that presented to a referral center in New York over a 3.5 year period for I131 therapy. Prior to therapy, cats had a procedure called “scintigraphy” preformed too allow calculation of total thyroid volume. This was done in order to calculate the dose of radioiodine needed, however also allowed for mapping of where thyroid nodules existed (unilateral, bilateral, or thoracic) and the size of these nodules.

Cats presented to these clinics were stratified into several groups based on duration of hyperthyroidism. Age of cats and sex were also recorded but did not significantly factor into analysis. Based on scintigraphy, cats were classified based on tumor pattern (unilateral, bilateral, or multifocal), location (cervical, thoracic inlet, or thorax), size (small, medium, large, or huge), ectopic locations (sublingual, cervical, mediastinal, or combinations), and whether the cats had a suspected carcinoma.

The authors of this study theorized that cats treated for hyperthyroidism over a long period with methimazole or iodine-restricted diets will see their adenomas gradually develop into carcinomas (malignant cancers).  Suspicion of carcinoma was based on tumor size, spread to ectopic locations, and pattern. A distinct sub-group of cats was identified with “SHIM-RAD” tumors. These were characterized by severe elevations to T4, huge tumor size, intrathoracic nodules, multifocal disease, and refractory to treatment with antithyroid drugs.

In the author’s findings, the prevalence of moderate to severe hyperthyroidism increased with duration of therapy. T4 concentration, thyroid size, presence of ectopic tissue, and prevalence of suspected carcinoma all increased with duration of disease. This progression was not surprising based on anecdotal findings, however the growth of new or ectopic tumors was not expected.

Some bias may be present in this study, as animals presented for I131 are often those in whom more conservative therapy has failed.

While it was not clear to the authors whether the increased severity of hyperthyroidism was due to the passage of time alone, or if methimazole therapy contributed to the progression of disease, it was evident that longer durations of treatment prior to I131 therapy were associated with more severe disease by most criteria.

It has not been determined what percentage of cats treated with medical anti thyroid therapy become non-responsive to treatment, however this research suggests that curative therapy such as I131 should be considered early in the course of disease. It also proposed a new category of “SHIM-RAD” tumors based on blood and scintigraphy results that may be indicative of progression to carcinoma. (MRK)

See also:
Peterson ME and Broome MR. Thyroid scintigraphy findings in 2,096 cats with hyperthyroidism. Vet Radiol Ultrasound 2015; 56: 84–95.