Syme, Harriet

Objectives: In cats with concurrent hyperthyroidism and non‐thyroidal illnesses such as chronic kidney disease, total thyroxine concentrations are often within the laboratory reference range (19 to 55 nmol/l). The objective of the study was to determine total thyroxine, free thyroxine and/or thyroid‐stimulating hormone concentrations in cats with mild chronic kidney disease. Methods: Total thyroxine, free thyroxine and thyroid‐stimulating hormone were measured in three groups. The hyperthyroidism‐chronic kidney disease group (n=16) had chronic kidney disease and clinical signs compatible with hyperthyroidism but a plasma total thyroxine concentration within the reference range. These cats were subsequently confirmed to be hyperthyroid at a later date. The chronic kidney disease‐only group (n=20) had chronic kidney disease but no signs of hyperthyroidism. The normal group (n=20) comprised clinically healthy senior (>8 years) cats. Results: In 4 of 20 euthyroid chronic kidney disease cats, free thyroxine concentrations were borderline or high (≥40 pmol/l). In the hyperthyroidism‐chronic kidney disease group, free thyroxine was high in 15 of 16 cats, while thyroid‐stimulating hormone was low in 16 of 16 cats. Most hyperthyroidism‐chronic kidney disease cats (14 of 16) had total thyroxine greater than 30 nmol/l, whereas all the chronic kidney disease‐only cats had total thyroxine less than 30 nmol/l. Clinical Significance: The combined measurement of free thyroxine with total thyroxine or thyroid‐stimulating hormone may be of merit in the diagnosis of hyperthyroidism in cats with chronic kidney disease.

Introduction and Objectives: Hyperthyroidism in cats, caused by nodular hyperplasia or adenomas, is clinically and histologically similar to toxic nodular goiter in humans. Subclinical hyperthyroidism in humans is defined as low thyrotropin (TSH) in conjunction with within-reference-range thyroid hormone concentrations, but has not previously been defined in cats. The objective of this study was to test the hypothesis that euthyroid senior cats with low TSH have histological evidence of thyroid nodular hyperplasia and/or adenoma. Design: Thyroid glands removed postmortem from four groups of cats (n = 73) were examined histologically and scored in a blinded fashion. Clinically euthyroid senior (>7 years) cats were divided into two groups dependent on their TSH concentration—TSH below the limit of quantification (LOQ) of the assay (<0.03 ng/mL; n = 15; UndetectableTSH group) and TSH above the LOQ (≥0.03 ng/mL; n = 31; DetectableTSH group)—using archived plasma samples, collected 0–6 months antemortem. Thyroids were also scored for two control groups: Young group (cats <6 years old; n = 13) and Hyperthyroid group (clinically and biochemically hyperthyroid cats; n = 14). Main outcome: Cats in the UndetectableTSH group had a higher frequency of nodules, a greater percentage of abnormal thyroid tissue, and a higher overall histopathological grade than cats with detectable TSH had. Conclusion: Euthyroid (as defined by total thyroxine) senior cats with low TSH are likely to have histological evidence of nodular thyroid disease, and such cats could be considered to be subclinically hyperthyroid.