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  • Author or Editor: Gregory MacEwen x
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SUMMARY

To determine the effects of long-term thyroxine treatment, histomorphometric analysis was performed on the pituitary and thyroid glands of healthy dogs, dogs treated for 9 weeks with a replacement dose of l-thyroxine, and dogs at 6 weeks after cessation of thyroxine treatment. In treated dogs, the volume density of thyrotropes decreased during thyroxine treatment and increased 6 weeks after cessation of treatment, compared with thyrotropes of healthy nontreated dogs. The activity of the thyroid gland was decreased in dogs during thyroxine treatment, as evidenced by decreases in epithelial volume density, epithelial height, and follicular area, and increase in colloid volume density, compared with thyroid gland activity in nontreated dogs. After cessation of thyroxine treatment, the thyroid gland had decreased colloid area, follicular area, and epithelial volume density, and increased interstitial volume density, compared with the thyroid gland of healthy nontreated dogs. Thyroxine treatment resulted in suppression of pituitary thyrotropes and thyroid follicular activity.

Free access
in American Journal of Veterinary Research

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.

Free access
in American Journal of Veterinary Research