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. 2021 Oct;23(10):928-935.
doi: 10.1177/1098612X20983973. Epub 2021 Jan 19.

Incidence of microcytosis in hyperthyroid cats referred for radioiodine treatment

Claudia Gil-Morales  1 , Marta Costa  1 , Kathleen Tennant  2 , Angie Hibbert  3
Affiliations

Incidence of microcytosis in hyperthyroid cats referred for radioiodine treatment

Claudia Gil-Morales et al. J Feline Med Surg. 2021 Oct.

Abstract

Objectives: The 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.

Methods: Retrospective 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.

Results: There 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 relevance: Microcytosis was present in a significant proportion of hyperthyroid cats, most without clinically significant comorbidities, and resolved in some following RAI.

Keywords: Hyperthyroidism; iron; microcytosis; radioiodine.

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Conflict of interest statement

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1
Figure 1
Flowchart demonstrating the short- and long-term outcome of hyperthyroid cats treated with radioiodine in relation to their microcytosis status. MCV = mean cell volume
See this image and copyright information in PMC

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