Abstract
ObjectivesThe study aimed to document the incidence of erythrocyte microcytosis in a population of hyperthyroid cats referred for radioiodine (RAI) treatment. Microcytosis has been observed but not described in feline hyperthyroid patients and is associated with hyperthyroidism in humans.MethodsRetrospective clinicopathological data were collected for cats undergoing RAI between January and December 2017. Microcytosis was defined as mean cell volume (MCV) <41.3 fl using the ADVIA 2120 haematology analyser (Siemens) and identified on blood smear examination by a haematology laboratory scientist or board-certified specialist in veterinary clinical pathology. Hyperthyroidism was classified as mild (total thyroxine [TT4] 60–124.9 nmol/l), moderate (TT4 125–250 nmol/l) or severe (TT4 ⩾251 nmol/l) immediately before RAI. Data were analysed descriptively and using a Pearson correlation coefficient to test the relationship between TT4 and microcytosis, and time elapsed between first diagnosis and MCV.ResultsThere were 41 female and 37 male cats with an age range of 7.2–20.8 years. Most cats were non-pedigree (98.7%). Microcytosis (median MCV 39.8 fl, interquartile range 32.3–41.2) was present in 29.5% (23/78) of the cats. Of the 23 microcytic samples, 86.9% (20/23) were confirmed as such on smear examination. Of mildly, moderately and severely hyperthyroid cats, 23% (6/26), 28.1% (9/32) and 40% (8/20) were microcytic, respectively. Two microcytic cats had low red blood cell counts (<6 × 1012/l) and low haemoglobin concentration (<8.2 g/dl). There was no correlation between TT4 or time elapsed from first diagnosis and MCV. Microcytosis resolved in 77.7% (7/9) of cases with follow-up. One microcytic cat had significant comorbidities (portosystemic shunt).Conclusions and relevanceMicrocytosis was present in a significant proportion of hyperthyroid cats, most without clinically significant comorbidities, and resolved in some following RAI.
Highlights
Hyperthyroidism is the most common endocrine disease of domestic cats, affecting especially middle-aged and senior cats.[1,2] Its classic clinical features were well described decades ago after hyperthyroidism was first reported in 1979,3 and since ongoing research has mainly focused on its diagnosis and treatment
Microcytosis was defined as mean cell volume (MCV)
A causal relationship was not established. This contrasts with the available literature regarding clinicopathological findings in feline hyperthyroidism, where macrocytosis has been previously reported in 31–45% of these cats.[1,2]
Summary
Hyperthyroidism is the most common endocrine disease of domestic cats, affecting especially middle-aged and senior cats.[1,2] Its classic clinical features were well described decades ago after hyperthyroidism was first reported in 1979,3 and since ongoing research has mainly focused on its diagnosis and treatment. Microcytosis, defined as the presence of small red blood cells compared with a reference population, can be observed with several diseases including iron deficiency, iron-restricted haematopoiesis (such as in hepatic disease, chronic kidney disease [CKD] or chronic inflammatory disease) and vitamin B6 deficiency
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