Long-term Outcome of a Chihuahua With Idiopathic Syndrome of Inappropriate Antidiuretic Hormone Secretion

An approximately 3 yr old female Chihuahua was presented for evaluation of polyuria and polydipsia noted by the owner after adoption. Serum biochemistry and urinalysis revealed hyponatremia, hypochloremia, hypo-osmolality, and normal fractional excretion of sodium. Serum antidiuretic hormone concentration was 5.54 pg/mL and serum copeptin concentration was 135.40 pg/mL. The antidiuretic hormone value was above the reported range for healthy dogs and was considered inappropriate in light of serum osmolality. Idiopathic syndrome of inappropriate antidiuretic hormone secretion was considered the most likely diagnosis after excluding other potential causes, such as diuretic use, renal disease, adrenal disease, and thyroid dysfunction. The dog remains clinically well 2 yr after diagnosis without specific treatment other than minimal dietary salt supplementation. This case is unique because of the absence of systemic or neurologic signs, good long-term outcome without treatment, and use of novel diagnostic testing.