Canine hypothyroidism: a retrospective study of 108 cases.

Hypothyroidism, defined as a serum concentration of less than 70 ng of triiodothyronine (T3)/dl or less than 1.5 microgram of thyroxine (T4)/dl, or both, was diagnosed in 108 dogs by means of radioimmunoassay techniques. Both T3 and T4 values were determined in 96 dogs. Both values were low in approximately 50% (47/96) of the hypothyroid dogs; 25% (24/96) were T3 hypothyroid (low T3, normal T4), and 26% (25/96) were T4 hypothyroid (normal T3, low T4). The T3 values varied markedly between animals. Common clinical signs were alopecia, changes in coat (dryness, dullness, dandruff, scales, coarseness, and slow regrowth of hair), hyperpigmentation, and pyoderma. Doberman Pinschers, Great Danes, Poodles, Schnauzers, Irish Setters, and Boxers accounted for 50% (54/108) of the hypothyroid dogs. Therapy consisted of T4, T3, or combination T3 and T4 replacement. A good clinical response was observed with T4 replacement in 40.4% (19/47) within 2 months and in 21.3% (10/47) within 5 months. A poor clinical response was observed in 25.5% (12/47) after 6 months of therapy. Posttherapeutic evaluation was based on peak serum concentrations of T3 and T4, pretreatment (basal) thyroid hormone values, type of replacement therapy, dosage and time of sampling after treatment, concurrent clinical problems, and, most important, clinical response to thyroid replacement therapy.