Influence of Systemic Nonthyroidal Illness on Serum Concentration of Thyroxine in Hyperthyroid Cats

McLoughlin MA.

J Am Anim Hosp Assoc 1993;29:227-234.

BACKGROUND: Clinical findings indicative of hyperthyroidism are less common in cats with concurrent severe nonthyroidal illness than in cats without concurrent nonthyroidal illness. Nonthyroidal illness (NTI) can lower serum thyroid hormone concentrations in hyperthyroid as well as euthyroid cats.

SUMMARY: Systemic nonthyroidal illness was identified in 39 of 110 hyperthyroid cats in a retrospective study. The 110 cats were divided into four groups. Group A had elevated serum T4 concentrations and no other diseases. Group B had elevated serum T4 concentrations and one or more NTI. Group C had normal-to-low serum T4 concentrations and no NTI. Group D had normal-to-low serum T4 concentrations with one or more NTI. Mean serum T4 concentration was significantly decreased in cats with NTI compared to hyperthyroid cats without NTI. Serum T4 concentrations were normal in three hyperthyroid cats without NTI and 11 cats with NTI. Seven of these 11 cats had multiple nonthyroidal illnesses, although the severity of illness was not graded. Hyperthyroidism was confirmed in these cats by radionuclide thyroid imaging or histologic examination of the thyroid glands. Specific illnesses likely to result in a reduction in serum T4 were not found, but chronic renal failure, chronic heart failure, and neurological diseases were most common. The authors concluded that NTI is associated with a significant decrease in serum T4 in hyperthyroid cats. Diagnostic methods other than solely the measurement of baseline serum T4 concentration should be undertaken in cats with clinical findings suggestive of hyperthyroidism and normal serum T4.

CLINICAL IMPACT: This study’s results show that NTI can be associated with normal serum T4 concentrations in cats with apparent hyperthyroidism. Some cases with normal T4 concentration and thyroid nodules may be preclinical conditions and discovered by serendipity. While many systemic illnesses could decrease serum T4 concentrations in normal or hyperthyroid cats, diabetes mellitus, hepatopathy, renal failure, and systemic neoplasia are most frequently associated with a decrease in serum T4 in euthyroid cats. As in euthyroid cats, it is likely that the severity of illness is more important than the specific illness. Identification of NTI in a cat in which a diagnosis of hyperthyroidism is suspected should lead to consideration of alternative methods of diagnosis such as nuclear scintigraphy, thyroid biopsy, or a T3 suppression test.

NOTE: This article summary and comments are from ‘Small Animal Clinical Endocrinology’ Vol 4[1], 1994, edited by C. B. Chastain, DVM, MS, DipACVIM; Dave Panciera, DVM, MS, Dip ACVIM; and Mollyann Holland, DVM and published by Daniels Pharmaceuticals