title: Effects of aging on the pituitary-thyroid axis in the dog.
abstract: The effects of advancing age on basal serum concentrations of thyroxine (T4) and thyrotropin (TSH), on T4 responses to TSH, and on TSH responses to thyrotropin-releasing hormone (TRH) were studied in beagle dogs. A total of 27 female dogs belonging to four age-groups were used: prepubertal (11.4 +/- 0.2 (SD) weeks), adult (2.1 +/- 0.3 years), middle-aged (6.5 +/- 0.2 years), and old (12.4 +/- 0.3 years). There was no significant difference between serum T4 concentrations of prepubertal (4.3 +/- 0.1 microgram/dl, mean +/- SE) and adult dogs (4.1 +/- 0.1 microgram/dl). Also, the decreases between adulthood and middle age (16.01%) and between middle age and old age (23.5%) were not significant, but serum T4 levels in the old dogs (2.6 +/- 0.2 micrograms/dl) were significantly (p less than 0.01) lower than those in the adult dogs. Serum cortisol levels showed a progressive increase with advancing age and were significantly higher in old animals (20.1 +/- 2.4 ng/ml) compared to those in the adults (13.4 +/- 1.2 ng/ml). There were no significant differences in serum TSH concentrations among the four age-groups. A single i.v. injection of TSH (0.15 IU/kg B.W.) raised serum T4 levels in the prepubertal animals by more than 45% in 1 h and by more than 100% in 2 h. T4 responses to TSH in the adult dogs were similar to those in the prepubertal dogs but were greatly delayed and subdued in the middle-aged and old animals. A single i.v. injection of TRH (5 micrograms/kg B.W.) increased serum TSH levels in the prepubertal animals by more than 120% in 15 min. These increases were less in the adult (74.8%) and middle-aged (21.3%) animals, and a significant increase (24.6%) in the old animals did not occur until 30 min after TRH treatment. This study demonstrates that, with advancing age, marked alterations occur in the regulation of the T4-TSH system in the dog.
authors.affiliation: Department of Anatomy and Physiology, Kansas State University, Manhattan 66506.
authors.forename: E
authors.initials: E
authors.lastname: Gonzalez
authors.forename: S K
authors.initials: SK
authors.lastname: Quadri
mesh_terms: D000375:Aging; D000818:Animals; D004285:Dogs; D005260:Female; D006854:Hydrocortisone / Q000097:blood; D007030:Hypothalamo-Hypophyseal System / Q000254:growth & development* / Q000378:metabolism; D013961:Thyroid Gland / Q000254:growth & development* / Q000378:metabolism; D013972:Thyrotropin / Q000097:blood / Q000378:metabolism / Q000494:pharmacology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology; D013974:Thyroxine / Q000097:blood / Q000378:metabolism
chemical_list: D013973:Thyrotropin-Releasing Hormone; D013972:Thyrotropin; D013974:Thyroxine; D006854:Hydrocortisone