Serum Thyroid Hormone and Thyrotropin Concentrations in Adult Horses as They Age


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Abstract

With more horses remaining active longer in life, it is important to characterize changes that occur normally with aging, so that these can be differentiated from development of disease. The objective of the study was to test the hypotheses that geriatric horses have lower circulating concentrations of thyroid hormones and/or higher serum thyrotropin (TSH) concentrations compared to younger horses. Serum thyroid hormone and TSH concentrations from 71 normal, healthy horses that had participated in prior research projects were analyzed for effects of age, sex, and season when samples were obtained. All samples had been assayed in the same previously validated radioimmunoassays. There were no differences in serum concentrations of thyroid hormones or TSH by sex or season. Serum total thyroxine (T4) was greater in 3- to 6-year-old horses compared to all other age groups and was negatively correlated with age. There were no differences among age groups for free T4 and total and free tri-iodothyronine (T3). Serum TSH concentration was significantly greater in old horses (≥15 or ≥20 years) compared to young (3–10 years) and intermediate (11–14 years) age groups. Serum TSH was positively correlated with age. There were no significant differences in thyroid hormone responses to thyrotropin releasing hormone (TRH) among young, intermediate, or old horses. However, the TSH response to TRH was significantly different in both groups of older horses compared to intermediate and young horses. Serum total thyroxine concentrations decrease and serum TSH concentrations increase in horses as they age, with no changes in free T4 or T3.HighlightsResting serum concentrations of total thyroxine (TT4) were greater in young horses (3–6 years old) compared to horses aged 7 to 10, 11 to 14, ≥15, or ≥20 years. There was a significant negative correlation of TT4 with age.There were no differences in resting serum concentrations of free thyroxine, total and free tri-iodothyronine among horses aged 3 to 6, 7 to 10, 11 to 14, ≥15, or ≥20 years. There were also no significant correlations of these thyroid hormones with age.Resting serum concentrations of thyrotropin (TSH) were significantly greater in older horses (either ≥15 years or ≥20 years) compared to younger horses. There was a significant correlation of TSH with age.Serum thyroid hormone responses to thyrotropin releasing hormone (TRH) were not different among horses aged 3 to 10, 11 to 14, ≥15, and ≥20 years.Serum TSH response to TRH was significantly different between young (3–10 years) and intermediate (11–14 years) horses compared to old horses (≥15 years or ≥20 years). The source of this difference was most likely mainly due to the difference in resting TSH concentrations because the peak increases in response to TRH were not different.

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