Adult-Onset Hypothyroidism in Cats May Be Underdiagnosed: Insights from 17 New Cases

Spontaneous adult-onset primary hypothyroidism in 17 cats

Gustavo C. Cobucci, Santiago Teyssandier, Flávia M. Tavares, Marcella M.F. Rosa, Federico Fracassi, Carlos A. Geraldo Júnior, Cristina Aparecida Dias Bueno, Mariana Palha de Brito Jardim, Flávia da Silva Lourenço, Diego Daniel Miceli, Jorge Daniel García, Elber Alberto Soler Arias, Mark E. Peterson

Background

Spontaneous primary hypothyroidism in adult cats is traditionally considered rare, with only a limited number of cases previously reported worldwide. Unlike congenital hypothyroidism, which presents in kittens with disproportionate dwarfism, adult-onset hypothyroidism typically manifests with vague and subtle clinical signs such as lethargy, dermatologic abnormalities, and weight gain. The underlying pathology in adults may include lymphocytic thyroiditis, idiopathic thyroid atrophy, or goitrous hyperplasia. This study aimed to expand current knowledge by describing the clinical features, laboratory findings, thyroid imaging characteristics, and treatment response in 17 adult cats diagnosed with spontaneous hypothyroidism across four countries over a 4-year period.

Methods

This multicenter, observational, ambispective case series included 17 adult cats (>1 year old) diagnosed between 2021 and 2025 in Brazil, the USA, Argentina, and Italy. Inclusion required documentation of low or low–normal serum total thyroxine (T4) concentrations with elevated thyroid-stimulating hormone (TSH), and confirmation of primary hypothyroidism via thyroid imaging (scintigraphy, ultrasonography, or CT). Data collected included signalment, clinical signs, laboratory findings, thyroid imaging results, levothyroxine dosing, biochemical responses, and clinical outcomes. Statistical analysis was performed using non-parametric methods.

Results

Cats ranged from 2 to 14 years of age (median 8 years), with no breed predilection and a slight male predominance. Common clinical signs included lethargy (76.5%), dermatologic abnormalities (52.9%), and weight gain (52.9%). Palpable goiter was identified in 41.2% of cats. The most frequent laboratory abnormality was azotemia (29.4%), followed by anemia and hypercholesterolemia. Thirteen cats had low total T4 concentrations, while all cats had elevated TSH. Imaging revealed thyroid atrophy in 58.8% and bilateral thyroid enlargement (goitrous hypothyroidism) in 41.2%.

Fifteen cats received levothyroxine therapy (median dose 133 µg/cat/day). Treatment led to increased serum T4, decreased TSH, improved alertness and activity, resolution of goiter, normalization or improvement of azotemia, and remission of concurrent diabetes mellitus in two cats. No adverse treatment effects were reported.

Limitations

Limitations include the observational design, multicenter variability in diagnostic assays and imaging modalities, incomplete follow-up in some cases, and lack of histopathologic confirmation of thyroid pathology. Only cats with biochemical confirmation of primary hypothyroidism were included, potentially excluding early or central forms of the disease. Therefore, findings are descriptive and hypothesis-generating rather than reflective of disease prevalence.

Conclusions

Spontaneous adult-onset hypothyroidism in cats may be underrecognized in clinical practice. Affected cats commonly present with nonspecific signs such as lethargy, dermatologic changes, weight gain, azotemia, or worsening diabetes mellitus. Diagnosis relies on identifying low or low–normal T4 with elevated TSH, with imaging distinguishing atrophic from goitrous forms. Levothyroxine therapy consistently improved clinical and biochemical abnormalities, including reversal of azotemia and diabetic remission in some cases. Clinicians should consider hypothyroidism in adult cats with unexplained dermatologic abnormalities, azotemia, weight gain, or poor glycemic control.

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