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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 Medical and Research Services Boston
Wadsworth Veterans Administration Hospitals Boston
Departments of Medicine, Tufts University School of Medicine Boston, Massachusetts 02130;
University of California School of Medicine Los Angeles, California 90073

Reprints: Dr. C. T. Sawin, Boston VA Hospital, Boston, Mass. 02130.

The response of serum TSH to duplicate tests with each of two doses of TRH (30 µg and 500 µg) 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 µg TRH was 2.7–19.5 µU/ml, those subjects (3 of 22) with a peak TSH value between 2 and 5 µU/ml on one occasion were all >5 µU/ml on another. Thus, despite the intra-individual variability, a peak TSH value after 500 µg TRH of <2 µU/ml indicates TSH deficiency and >5 µU/ml indicates normal TSH reserve. A peak value of 2 – 5 µU/ml is an indication for retesting; a peak TSH value >5 µU/ml on retesting indicates normal TSH reserve.

The use of the maximal increase in TSH above basal values (max TSH) did not have a clear advantage over the use of the peak TSH value although a max TSH >4 µU/ml was equivalent to a peak value >5 µU/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 in measured serum values.

The overall TSH response to 500 µg TRH was statistically greater than the response to 30 µg TRH (P < 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 fig and 500 µg TRH, is quite shallow. The TSH value 60 min after 500 µg TRH was within 2 µU/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 < 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<0.02), suggesting that the concentration of serum T₄ within the normal range is a determinant of TSH secretion.

Supported by VA research grants 3590-02 and 7716-02 and USPHS grant HD-7181.

Received July 14, 1975.

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