SUMMARY
The effects of treatment with l-thyroxine (1 mg/m2 of body surface/d, po, for 8 weeks) on the thyroxine (T4) and triiodothyronine (T3) responses to thyrotropin (tsh) and thyrotropin-releasing hormone (trh) administration were determined in 10 euthyroid Beagles; 4 other dogs acted as controls. The tsh response test was performed before treatment and at weeks 2, 4, and 8 of treatment in all dogs and at 2 and 4 weeks after cessation of treatment in 6 dogs. The trh response test was performed before treatment and at week 6 of treatment in all dogs and at 5 weeks after cessation of treatment in 6 dogs.
Suppression of the T3 response to tsh was evident at treatment week 2, whereas the T4 response was suppressed at week 4 and remained suppressed for the duration of the study. Four weeks after l-thyroxine treatment was stopped, T3 response to tsh had returned to pretreatment values. Four weeks after stopping treatment, T4 and T3 responses to tsh in 2 dogs were within the hypothyroid range. The T4 response to trh was completely suppressed after 6 weeks of thyroxine treatment, but returned to pretreatment values by 5 weeks after cessation of treatment. Suppression of thyroid and pituitary function is evident after administration of a replacement dose of l-thyroxine to euthyroid dogs.
