Effects of thyrotropin-releasing hormone on serum concentrations of thyroxine and triiodothyronine in healthy, thyroidectomized, thyroxine-treated, and propylthiouracil-treated dogs

Effects of thyrotropin-releasing hormone (TRH) on serum concentrations of thyroid hormones were studied in 36 mixed-bred dogs. Dogs were randomly assigned to 7 groups. Significant increases (P less than 0.05) of serum thyroxine (T4) values occurred as early as 2 hours and reached a peak at 6 to 8 hours after IV injection of 300 to 1,100 micrograms of TRH. Thyroxine concentrations in response to a TRH dose greater than 500 micrograms were similar to those observed with the 300-micrograms dose. Transient coughing, vomiting, salivation, and defecation after large doses (900 and 1,100 micrograms) were observed. Mean serum T4 concentration decreased from 2.1 micrograms/dl to 0.9 micrograms/dl within 1 day of thyroidectomy. Clinical signs of hypothyroidism, including lethargy, dry coats, and diffuse alopecia, were present in 2 dogs at a month after surgical operation. Thyroxine concentrations were detectable for greater than 2 months. Injection (IV) of 700 micrograms of TRH 6 weeks after surgical operation had no effect on serum concentration of T4 in thyroidectomized dogs. In 5 T4-treated dogs, TRH (700 micrograms, IV) significantly increased the serum T4 value, indicating that pituitary thyrotropes were responsive to TRH, in spite of daily medication of 0.8 mg of T4. Four dogs were treated orally with 200 mg of propylthiouracil/day for 5 weeks. Intravenous injection of 700 micrograms of TRH in propylthiouracil-treated dogs had no effect on the serum T4 concentration, indicating that TRH had no effect on serum T4 values in these dogs during the experimental period. These results indicate that TRH can replace bovine thyrotropin for the canine thyroid function test.(ABSTRACT TRUNCATED AT 250 WORDS)