Primary goitrous hypothyroidism in a young adult domestic longhair cat: diagnosis and treatment monitoring

Abstract

Case summaryPrimary goitrous hypothyroidism was diagnosed in a 12-month-old cat examined because of small stature, mental dullness, severe lethargy, generalized weakness and gait abnormalities. Radiographs of the long bones and spine revealed delayed epiphyseal ossification and epiphyseal dysgenesis. Diagnosis of primary hypothyroidism was confirmed by low serum concentrations of total and free thyroxine (T4) with high thyroid-stimulating hormone (TSH) concentrations. Thyroid scintigraphy revealed severe enlargement of both thyroid lobes, as evidenced by a seven-fold increase in calculated thyroid volume above the reference interval. In addition, this bilateral goiter had an extremely high radionuclide uptake, about 10-fold higher than the normal feline thyroid gland. Treatment with twice-daily levothyroxine (L-T4), administered on an empty stomach, resulted in increased alertness, playfulness, strength and improvement in gait, as well as an increase in body length and weight. L-T4 replacement also led to normalization of serum thyroid hormone and TSH concentrations, and complete resolution of goiter.Relevance and novel informationSpontaneous hypothyroidism is rarely reported in cats, with congenital hypothyroidism in kittens diagnosed most frequently. Despite the fact that this cat was a young adult, it likely had a form of congenital hypothyroidism caused by dyshormonogenesis (defect in thyroid hormone synthesis) that led to compensatory development of goiter. In hypothyroid cats, treatment with L-T4 is best given twice daily on an empty stomach to ensure adequate absorption. Normalization of serum TSH and shrinkage of goiter, as well as improvement in clinical signs, is the goal of treatment for cats with goitrous hypothyroidism.

Spontaneous primary hypothyroidism in 7 adult cats

“…The hypothesis that best fits our 6 cats with goitrous hypothyroidism is that these cats suffer from a hereditary defect or block in thyroid hormone production (dyshormonogenesis) by an anatomically intact thyroid gland . This would be similar to the defects reported in kittens with congenital goitrous hypothyroidism, which is most commonly associated with an impaired ability of the thyroid gland to organify iodide (ie, organification or peroxidase defect) . In congenital, goitrous hypothyroidism, the inability to secrete adequate amounts of T 4 and T 3 leads to the loss of normal negative‐feedback inhibition on pituitary thyrotropes, resulting in persistent secretion of excessive amounts of TSH.…”

“…Such milder or partial defects in thyroid hormone production could explain the pathophysiology in our 6 goitrous cats, in which goiter was one of the major reasons for work‐up and few overt clinical signs of thyroid disease were present. If this is the case, our cats were able to compensate for the block in thyroid hormone secretion for many months to years, as evidenced by the cats’ normal rate of growth, body size, and closure of bone physes (ie, none of our adult cats were dwarfed or had open bone plates, both characteristic finding in kittens with congenital hypothyroidism) . In addition, none of the 6 cats with goitrous hypothyroidism displayed signs of mental dullness, as reported in younger cats with untreated congenital hypothyroidism .…”

“…In humans, the clinical presentation of patients with dyshormonogenetic goiter depends on the severity of the inborn error in thyroid hormone metabolism. A severe defect will lead to neonatal or congenital hypothyroidism, goiter, mental retardation, and growth abnormalities (cretinism), similar to many of the reported hypothyroid kittens . Milder forms of congenital dyshormonogenesis can present later in life (ie, adolescence or young adulthood) as goiter and minimal, if any, signs of thyroid dysfunction .…”

“…Most hypothyroid cats are younger (generally kittens, aged 2–4 months) and suffer from congenital hypothyroidism, which typically results in disproportionate dwarfism (cretinism). Of ∼60 cats with congenital hypothyroidism that have been reported, only 3 were older than 12 months of age at time of diagnosis, but these 3 cats were stunted since kittenhood, consistent with the diagnosis of congenital hypothyroidism.…”

“…All cats underwent thyroid scintigraphy by injecting 111 MBq of sodium 99m Tc‐pertechnetate ( 99m ) IV and imaging 1 hour later, as previously described . Thyroid activity was quantified by calculation of both the thyroid‐to‐salivary gland ratio (T/S) and the percent thyroidal uptake of the injected 99m (TcTU) . The estimated thyroid volume was also calculated from the scintigraphic image, using the equation for a spheroid as previously described .…”