The TSH response to thyrotropin-releasing hormone (TRH) in young adult men: intra-individual variation and relation to basal serum TSH and thyroid hormones.

The response of serum TSH to duplicate tests with each of two doses of TRH (30 mug and 500 mug) was studied in 22 normal young adult men. The mean intra-individual variability of the response assessed by duplicate testing (coefficient of variation) was 17% but was as high as 63% in individual subjects. While the actual range of peak TSH values after 500 mug TRH was 2.7-19.5 muU/ml, those subjects (3 of 22) with a peak TSH value between 2 and 5 muU/ml on one occasion were all greater than 5 muU/ml on another. Thus, despite the intra-individual variability, a peak TSH value after 500 mug TRH of less than 2 muU/ml indicates TSH deficiency and greater than 5 muU/ml indicates normal TSH reserve. A peak value of 2-5 muU/ml is an indication for retesting;; a peak TSH value greater than 5 muU/ml on retesting indicates normal TSH reserve. The use of the maximal increase in TSH above basal values (max deltaTSH) did not have a clear advantage over the use of the peak TSH value although a max deltaTSH greater than 4 muU/ml was equivalent to a peak value greater than 5 muU/ml. No information was lost by using only the TSH value at 30 min after TRH instead of multiple samples. In using these values differences in assay technique should be considered; for example, the use of human TSH standard MRC 68/38 instead of human TSH standard A (MRC 63/14) causes a fall of about 1/3 in measured serum values. The overall TSH response to 500 mug TRH was statistically greater than the response to 30 mug TRH (P less than 0.01); however, in 10 of 22 subjects the response to the two doses was about the same, suggesting that the dose response of TSH to TRH, between 30 mug and 500 mug TRH, is quite shallow. The TSH value 60 min after 500 mug TRH was within 2 muU/ml of the peak TSH value in 12 of 22 subjects on at least one occasion; this pattern of a delayed fall is a normal variant. The peak TSH response to TRH correlated well with the basal level of TSH (P less than 0.001) and thus can be considered a magnifier of the basal level of TSH in normal subjects. While the peak TSH value did not correlate with the basal level of T3, there was a moderate negative correlation of the peak TSH value with the basal level of T4 (P less than 0.02), suggesting that the concentration of serum T4 within the normal range is a determinant of TSH secretion.