Congenital hypothyroidism in a dog due to an iodide organification defect

A 10-month-old dwarfed male pup born of large-breed parents with normal stature had congenital goiter. Hypothyroidism due to failure to form organic iodine was diagnosed based on low serum thyroxine and triiodothyronine assays, radiographic findings in the epiphyses, and an abnormal perchlorate discharge test. Replacement of thyroid hormone resulted in more than 450% increase in body weight and a 600% increase in shoulder height over an 8-month period of treatment. A relapse in mental and physical condition occurred when hormone replacement therapy was later withdrawn. Postmortem findings are described.