The role of this hormone in diagnosing hyperthyroidism is an area of active research in veterinary medicine.
Subnormal concentrations of thyroid stimulating hormone (TSH) along with an elevation in free thyroxine concentration is the primary method of diagnosing hyperthyroidism in people. The role of TSH in the diagnosis of cats with this disease, however, is still an area of active research in veterinary medicine.
A single TSH concentration below 0.03 ng/ml is a strong predictor for the development of hyperthyroidism. (Todd Gipstein/Getty Images)
Previous studies have demonstrated that cats with a low TSH concentration may indeed have a form of subclinical hyperthyroidism that can ultimately become overt disease. Researchers at the Royal Veterinary College in London conducted a prospective, cohort study evaluating whether low TSH concentrations in a population of geriatric cats would eventually lead to a diagnosis of hyperthyroidism and what, if any, clinical signs developed. Results of this study were published in a recent issue of the Journal of Veterinary Internal Medicine.
The population consisted of cats over 9 years of age that were presented for routine evaluation and with no apparent clinical problems or history of chronic illness or medications. Cats were excluded if a diagnosis of hyperthyroidism was made at the time of the initial visit or within 3 months of enrollment. The diagnosis of hyperthyroidism was based on a serum total T4 > 55 nmol/L (range 19-55 nmol/L), a serum total T4 > 40 nmol/L and free T4 > 40 pmol/L (range 19-40 pmol/L) in a cat with concurrent illness and supportive signs of hyperthyroidism, or by T3 suppression test. A complete physical examination, systolic blood pressure measurement (by Doppler), thyroid/TSH concentrations, PCV, urinalysis, and baseline serum chemistry testing were obtained at the initial visit. Cats with no underlying health problems at the initial visit were re-evaluated every 6 months. Those with evidence of azotemia, urinary tract infections, or hypertension were monitored as clinically necessary. All cats were followed for a minimum of 2.5 years and up to 4.5 years.
A total of 104 cats were enrolled. Thyroid status, TSH concentrations, and baseline variables were re-evaluated between 9 and 14 months of enrollment (short-term follow-up endpoint)and again between 2.5 to 4.5 years post-enrollment (long-term follow-up endpoint). Age, breed, sex, history of weight loss, vomiting, diarrhea, polyphagia, polydipsia, skin disease, behavioral changes, presence of a goiter, blood pressure, heart rate, and presence of a murmur were evaluated for a possible association with the development of hyperthyroidism.
At the time of the short-term follow-up, 85 cats remained in the study and, of these, 11 were diagnosed with hyperthyroidism. While baseline free T4 concentrations were similar between the hyperthyroid and nonhyperthyroid groups, the baseline TSH concentrations were significantly lower (<0.03 ng/ml) for those in the hyperthyroid group. Cats in this group were also more likely to have a goiter, a heart murmur, a history of vomiting, and a higher alkaline phosphatase; however, only a subnormal TSH concentration was determined to be predictive of the development of hyperthyroidism within 14 months. Of the cats with a subnormal TSH concentration at enrollment, 40% went on the develop hyperthyroidism. Only 1 cat with a detectable TSH concentration at the time of enrollment was diagnosed with hyperthyroidism at the short-term follow-up visit.
Long-term follow-up evaluation took place between 2.5 and 4.5 years following enrollment. An additional 6 cats were diagnosed with hyperthyroidism during this phase of the study for a total of 17 hyperthyroid cats. All of these cats also developed signs associated with hyperthyroidism (weight loss, tachycardia, vomiting, diarrhea, polyphagia). Thirteen of the 17 cats had undetectable TSH concentrations (< 0.03 ng/ml) at the baseline visit. For the 4 with detectable TSH concentrations at enrollment, all went on to demonstrate a decline in their TSH to < 0.03 ng/ml within 12 months with a subsequent diagnosis of hyperthyroidism within 6 to 28 months.
One of the limitations of using TSH concentrations in the evaluation of thyroid status in cats is the insensitivity of the assay at very low TSH concentrations. Almost one-third of recruited cats had a TSH concentration < 0.03 ng/ml at baseline and yet only 13 of 25 were diagnosed with hyperthyroidism during the study period. While development of a more sensitive assay will help determine the prognostic value of a single TSH concentration, results of this study still demonstrate that a single TSH concentration below 0.03 ng/ml is a strong predictor for the development of hyperthyroidism. Furthermore, results suggest that a TSH concentration above 0.03 ng/ml may serve as a marker to rule out hyperthyroidism in a geriatric cat.
Wakeling J, Elliott J, Syme H. Evaluation of predictors for the diagnosis of hyperthyroidism in cats. J Vet Intern Med 2011;25(5):1057-1065.
Veterinary Diagnostic Investigation and Consultation (VDIC)— David S. Bruyette, DVM, DACVIM, and Jennifer L. Garcia, DVM, DACVIM—has over 40 years of experience in internal medicine and endocrinology.