Congenital hypothyroidism has traditionally been thought of as a rare feline disease, however it is increasingly commonly recognized. It is caused by an inborn defect in thyroid hormone production, and may or may not have a goiter present. Hypothyroid cats often have stunted growth with open physes, constipation, craniofacial abnormalities, and neurologic deficits. It is generally treated with thyroid hormone supplementation.
The purpose of this paper was to describe a series of cases of spontaneous hypothyroidism diagnosed at a single referral institution in an 18-month period from 2017 to 2018. Cats were included if they had clinical signs supportive of hypothyroidism, a low Total T4, and an elevated cTSH. Eighteen hundred feline cases were seen in the study period, of which six met inclusion criteria.
The most commonly reported findings in affected cats included a small stature and lethargy or mental dullness. Disproportionate dwarfism with a large head, short and wide neck, flat face, short legs with a wide body, bradycardia, hypothermia, retained deciduous teeth, and obesity also occurred. Two cats had palpable goiters.
After diagnosis, all cats received therapy with oral levothyroxine at a median dose of 13.5mg/kg BID (range 10-20mg/kg). Blood TT4 and cTSH levels were monitored at 3 to 4 week intervals and doses titrated up to a median of 16mg/kg (range 14-20mg/kg). All owners reported an improvement in haircoat and physical activity. Heart rate, temperature, and physical appearance (including facial structure, stature and dentition) normalized by sixteen weeks post treatment. All cats had a significant decrease in thyroid volume.
Two years after starting therapy, 5 of 6 cats remained clinically stable and well controlled, while one cat had died of unrelated issues (trauma).
This case series serves to describe a previously “rare” disease in cats. It serves to demonstrate that this may be more common than previously suspected, and that cats who are diagnosed and treated have good long-term outcomes, even when diagnosed as adults. While a suspected diagnosis of goiterous hypothyroidism due to dyshormonogenesis was reached, thyroid histopathology was not performed in any case.
Some limitations were present in this study. These include its retrospective, single center nature with a possible lack of generalizability as a result, the relatively small number of clinical cases, and a lack of long term follow up (ie greater than two years). (MRK)