Why are the sensitive TSH assay and TRH stimulation test frequently low in very old euthyroid subjects?

Twelve healthy clinically euthyroid subjects 80 years and over were compared with 19 young controls to determine if the sensitive TSH test was more specific for hyperthyroidism in the elderly than the TRH stimulation test. Four of the 12 elderly subjects had a sensitive TSH level below the lower limit of normal for young subjects and four elderly subjects had a TSH rise of less than five IU after injection of 200 micrograms of TRH. Neither test is specific for hyperthyroidism in the elderly. There was a highly significant correlation between the sensitive TSH levels and the TSH rise after TRH stimulation in the old subjects but not in the young. This may be further evidence of an age related impairment in the hypothalamo pituitary-thyroid axis.