Abstract
In our previous study, we observed a tendency towards an age-related increase in the serum thyrotropin (TSH) concentration. Regulatory mechanisms TSH secretion in elderly subjects were studied. In 43 elderly subjects, serum TSH did not correlate significantly with serum T4, T3 free T4 or rT3. Further, those with increased TSH (>5 mU/l, 9 subjects) did not overlap with those with low T3 (<0.92 nmol/1, 8 subjects). Increases in serum TSH were not associated with the presence of circulating anti-thyroid autoantibodies.
A TRH test using a 500μg single bolus injection was performed in 15 subjects. TSH response (basal: 1.92±1.42 (s. d.) mU/1, peak: 11.25±5.33 mU/1, Σ: 26.74±12.89mU/1, respectively) did not differ significantly from that of younger subjects. T3 response after TRH varied greatly and a close correlation was observed between basal T3 and peak T3 (r=0.86), and also between peak T3and peak ΔT3 (r=0.81). A significant correlation was observed between ΣTSH and basal T3 (r=0.60). Neither plasma cortisol, epinephrine norepinephrine concentrations showed any significant correlation with basal and TRH-stimulated TSH or T3concentrations. However, the plasma dopamine concentration correlated significantly with ΣTSH (r=0.60) and basal T3 (r=0.52), respectively.
In conclusion, the increase in serum TSH observed in elderly subjects was felt to represent a physiological adaptation to maintain serum T3. Low T3subjects appear to have a disturbance in this mechanism, with decreased TSH and T3 response to TRH stimulation. The details of this disturbance havenot been clarified, but a possible dopaminergic mechanism was suggested by the decreased plasma dopamine concentration observed in the low T3 group.
