Dr. Peterson enjoyed lecturing at the Minnesota Veterinary Medical Association's 115th Annual Meeting last week on February 2, 2012. He lectured to a packed house, and he has already received positive feedback from attending veterinarians.
Dr. Peterson was recently appointed as the newest Executive Board member of the Association for Pet Obesity Prevention. This new appointment comes on the heels of Dr. Peterson's recent in-depth analysis of small animal nutrition and the role the pet food industry plays in what we feed our dogs and cats. To read more, click here.
Dr. Peterson is preparing for his lectures at the Western Veterinary Conference, which begins in a couple of weeks. His four lectures will cover disorders of polydipsia and polyuria, endocrine alopecias, hypercalcemia and hyperaldosteronism.
• Nutritional Management of Feline Hyperthyroidism
Endocrine Case of the Month:
Millie, a 9-year old female spayed DSH cat, was examined by her veterinarian for moderate weight loss despite an increase in appetite. She had developed a chronically loose stool over the past 3 months, and the owner thought that she had begun drinking and urinating more. No vomiting, coughing, sneezing or other respiratory signs had been noted.
On physical examination, Millie was bright, alert, and active. She was slightly thin (body condition score was 2 out of 5), with mild muscle wasting. The heart rate was 220 bpm, but no cardiac murmur or arrhythmia were auscultated. Her abdomen palpated normally. On cervical examination, no thyroid nodules were palpated.
Results of Millie's clinical pathology panel, which included a complete blood count, serum chemistry panel, complete urinalysis, and fecal analysis, were all normal. Renal disease was excluded as a cause of Millie's polyuria on the basis of normal serum concentrations of both creatinine (0.8 mg/dl) and urea nitrogen (24 mg/dl), together with a highly concentrated urine specific gravity (1.055).
A serum thyroid panel revealed a high-normal serum T4 concentration (3.2 μg/dl; reference range, 0.8-4.0 μg/dl). A free T4 concentration (by dialysis) was slightly high at 62 pmol/L (reference range, 10-50 pmol/L). Based upon these thyroid results, a presumptive diagnosis of mild hyperthyroidism was made, and Millie was started on a low dose of methimazole (1.25 mg twice daily).
Within the first 10 days of treatment, the owner noticed that Millie had developed intense facial pruritus, with subsequent excoriation on her face and neck (Figure 1). A methimazole reaction was suspected, and the drug was discontinued. Within a few days, Millie's pruritus had resolved and the facial lesions were healing (Figure 2).
Based upon Millie's young age and inability to tolerate methimazole, the owner elected for radioiodine treatment and was referred to the Hypurrcat division of the Animal Endocrine Clinic for treatment of her hyperthyroidism.
Further Workup and Management
On my physical examination, Millie was thin but otherwise normal. Like the referring veterinarian, I was unable to palpate a thyroid nodule.
Because of the lack of thyroid enlargement on cervical palpation, a thyroid scan was performed on Millie to verify the diagnosis of mild hyperthyroidism. After administering a short-acting radionuclide that concentrates in thyroid tissue, thyroid imaging directly visualizes the normal thyroid gland, as well as the small tumor(s) responsible for hyperthyroidism in cats. Because this procedure utilizes the physiology of the thyroid gland to create an image, thyroid imaging is so sensitive that it can actually demonstrate the presence of hyperfunctional thyroid tumors long before they become clinically significant or result in laboratory value abnormalities.
In normal cats, the thyroid gland appears on thyroid scans as two well-defined, focal (ovoid) areas of radionuclide accumulation in the cranial to middle cervical region. The two thyroid lobes are symmetrical in size and shape and are located side by side. Activity in the normal thyroid closely approximates activity in the salivary glands, with an expected “brightness” ratio of 1:1.
Based on these criteria, Millie's thyroid scan was completely normal (Figure 3). Her thyroid:salivary ratio was not increased, and we determined that Millie was not hyperthyroid. We repeated her total and free T4 concentrations, and this time both were found to be well within the respective reference range limits.
Millie was sent home on Purina EN Gastroenteric Feline Formula. Within the next 2 weeks, Millie's diarrhea completely resolved. On recheck 2 months later, she had regained her lost weight and her body condition score had normalized.
Comment
Hyperthyroidism is caused by one or more small, usually benign, tumors in the thyroid gland. These tumors function autonomously to produce the high circulating thyroid hormone concentrations responsible for hyperthyroidism. Diagnosing hyperthyroidism is usually straightforward. Generally, documenting a high serum T4 concentration in a cat with compatible historical and physical examination findings is sufficient to confidently diagnose hyperthyroidism.
Unfortunately, not all cats with hyperthyroidism will have a high T4 concentration. Sometimes, cats with early or mild hyperthyroidism have normal or only slightly high serum T4 concentrations. As a result, other tests have been designed to aid in the diagnosis of hyperthyroidism in cats with early or mild disease. Currently, the non-protein bound or “free” T4 is considered the most sensitive laboratory test for diagnosing mild hyperthyroidism in cats. Unfortunately, this test is not perfect either.
Thyroid scintigraphy or thyroid scanning has long been considered the “gold standard” for diagnosing feline hyperthyroidism. Thyroid imaging is so sensitive that it can actually demonstrate the presence of hyperfunctional thyroid tumors long before they become clinically significant or result in laboratory value abnormalities. At the Animal Endocrine Clinic, we have the nuclear medicine imaging equipment and special licensing required to perform thyroid scintigraphy. This allows us to readily perform thyroid imaging in any cat that needs it to accurately confirm the diagnosis of hyperthyroidism. This is especially helpful in cats like Millie that have borderline high T4 or free T4 concentrations but no palpable thyroid nodule.
This case is not that uncommon. Between 6% and 12% of all cats with nonthyroidal illness will have a falsely-high free T4 value and will be proven NOT to be hyperthyroid by thyroid scintigraphy. However, because these cats are not hyperthyroid, treatment will be of no benefit and will likely result in hypothyroidism.
Q & A: Diabetes Insipidus in a Hypothyroid Golden Retriever My patient is a 7-year old, male neutered Golden Retriever-mix weighing 69 pounds who was diagnosed as being hypothyroid and has been on L-thyroxine replacement (Soloxine) for at least 5 years.
The Animal Endocrine Clinic is the only clinic of its kind in the country that specializes in the diagnosis and treatment of cats and dogs with endocrine disorders. Dr. Peterson has set up two NY clinics (Manhattan and Bedford Hills) to service clients from New York City, Long Island, Westchester County, New Jersey and Connecticut.
This Clinic is separated into three divisions: the Endocrine Clinic, dedicated to diagnosing and treating dogs and cats with endocrine disorders; the Hypurrcat treatment center designed for treating hyperthyroid cats with radioactive iodine (I-131); and Nuclear Imaging for Animals, a state-of-the-art medical imaging facility where we use radioactive tracers to perform nuclear scanning (scintigraphy) for diagnosing of thyroid, bone, liver, and kidney diseases in dogs and cats.
The Animal Endocrine Clinic is a referral-only hospital, and does not offer prophylactic or routine care. We can be reached by phone at (212) 362-2650 or (914) 864-1631; by email at info@animalendocrine.com or on the web at www.animalendocrine.com.