Previous studies have established that thyroid hormone concentrations are higher in healthy young and adolescent animals, and lower in geriatric animals. Similarly, healthy toy and small breed dogs have higher metabolic rates and higher basal thyroid concentrations than large or giant breed dogs. Sighthounds as a group have lower resting thyroid hormone concentrations, and values in healthy sighthounds often fall just below the laboratory reference ranges.

Athletic conditioning or endurance exercise is another variable that affects thyroid hormone concentrations in healthy dogs. Significant decreases in T4 and freeT4 concentrations (> established references ranges in 11 of 19 and 8 of 19 dogs, respectively), and significant increases in TSH concentrations were consistently found for dogs in the peak training state for athletic events (sled dog racing) as compared with concentrations in the untrained state. Thus, endurance training has a profound impact on thyroid hormone concentrations in competitive sled dogs. Similar findings have been observed for greyhounds during racing season, dog conditioned for competition lure coursing.

References: Wolford et al., Vet Clin Pathol 17:35-42, 1988; Dodds, Adv Vet Sci Comp Med 39:29-96, 1995; Gaughan and Bruyette, Am J Vet Res 62:1130-1133, 2001; Hill et al., Am J Vet Res 62:1969-1972, 2001; Evason et al., Am J Vet Res 65:333-337, 2004