A longitudinal study of serum concentrations of thyroxine and triiodothyronine in the BUF rat

Abstract

Thyroid function was investigated in a colony of BUF inbred rats in order to establish the normal baseline in this model for spontaneous autoimmune thyroiditis, and to thereby attempt to monitor the development of the disease (histologic examination of a small sampling of the BUF rats confirmed that thyroiditis did occur in animals of this colony). Sera were obtained by longitudinal sampling of males and virgin females, and concentrations of thyroxine (T4) and triiodothyronine (T3) were measured by radioimmunoassays. T4 and T3 levels were each found to be correlated with body weight in males, with r = 0.264 (P < 0.002) and r = 0.261 (P <; 0.005) respectively, but neither hormone was significantly correlated with body weight in virgin females. T4 and T3 levels declined with age in both sexes; in males, r = -0.309 (P < 0.001) for T4 and r = -0.369 (P < 0.001) for T3, and in virgin females r = -0.283 (P < 0.001) for T4 and r = -0.417 (P < 0.001) for T3. Although these correlation coefficients do no indicate that there was a significant sexual difference in the variation with age of either T4 or T3 (P > 0.05 for each hormone), in general the females had slightly lower levels of T4 and slightly higher levels of T3 than did the males. For those samples that had been assayed for both hormones, the T4/T3 ratios were significantly (P < 0.001) lower in the females and were independent of age in both sexes. Serial samples revealed that T4 and T3 concentrations fluctuated greatly from month to month in animals of each sex. Values which fell outside of the normal ranges (95% confidence limits around the regression lines) could not necessarily be attributable to thyroiditis, with values from subsequent ages often returning to within the normal ranges.