Age related correlation between serum TSH and thyroid C cell hyperplasia in Long-Evans rats

The purpose of the present investigation was to determine whether there is a correlation between the serum levels of thyroid stimulating hormone (TSH), parafollicular cell (C) population in the thyroid and calcitonin (CT) secretion in aging Long-Evans (L-E) rats. Serum TSH and CT values were determined in 50 male rats at ages 2, 6, 12, 18 and 24 months and 50 female rats at 18, 21 and 24 months. The animals were sacrificed at the end of the experiment to evaluate thyroid C cell pathology. Male rats showed an increase in the serum TSH values at 6 and 12 months of age, a decline at 18 months and a significant increase from 18 to 24 months of age. In female rats, a significant increase in serum TSH concentration was also noted from 18 to 24 months of age. In both sexes, 24 month old animals with thyroid C cell hyperplasia (CH+ group) had a significantly higher level of serum TSH as compared to animals with normal distribution of C cells (CH- group). Concentration of serum CT showed a progressive increase with age in both sexes. Male rats with thyroid C cell pathology had significantly higher levels of serum CT at 24 months of age as compared to rats with normal C cell distribution. In female rats, however, serum CT concentrations in two groups were not statistically significant. We conclude from these studies that in aged L-E rats, serum TSH concentration has an influence on thyroid C cell population.