Effects of age, sex, and body size on serum concentrations of thyroid and adrenocortical hormones in dogs.

Thyroxine (T4), 3,5,3'-triiodothyronine (T3), and cortisol frequently are quantified in canine serum or plasma samples to aid in the diagnosis of hypothyroidism, hypoadrenocorticism, and hyperadrenocorticism. Many laboratories have established reliable references values for concentrations of these hormones in blood of clinically normal animals. However, nonpathologic factors that affect thyroidal and adrenocortical secretion may lead to misinterpretation of test results when values for individual animals are compared with reference values. The objective of the study reported here was to identify effects of age, sex, and body size (ie, breed) on serum concentrations of T3, T4, and cortisol in dogs. Blood samples were collected from 1,074 healthy dogs, and serum concentrations of the iodothyronines and cortisol were evaluated for effects of breed/size, sex, and age. Mean (+/- SEM) serum concentration of T4 was greater in small (2.45 +/- 0.06 micrograms/dl)- than in medium (1.94 +/- 0.04 micrograms/dl)- or large (2.03 +/- 0.03 micrograms/dl)-breed dogs, the same in females (2.11 +/- 0.04 micrograms/dl) and males (2.08 +/- 0.04 micrograms/dl), and greater in nursing pups (3.04 +/- 0.05 micrograms/dl) than in weanling pups (1.94 +/- 0.05 micrograms/dl), rapidly growing dogs (1.95 +/- 0.04 micrograms/dl), and young adult (1.90 +/- 0.06 micrograms/dl), middle-aged adult (1.72 +/- 0.05 micrograms/dl), or old adult (1.50 +/- 0.05 micrograms/dl) dogs. Dogs greater than 6 years old had lower mean serum T4 concentration than did dogs of all other ages, except middle-aged adults. Mean serum T3 concentration in medium-sized dogs (1.00 +/- 0.01 ng/ml) was greater than that in small (0.90 +/- 0.01 ng/ml)- and large (0.88 +/- 0.01 ng/ml)-breed dogs.(ABSTRACT TRUNCATED AT 250 WORDS)