FAILURE OF TSH RADIOIMMUNOASSAY TO CONSISTENTLY DETECT MILD TSH MEDIATED THYROID STIMULATION

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Abstract
SUMMARY: The sensitivity of the radioimmunoassay for serum TSH has been tested against a direct method for assessment of thyroid function. Fifteen healthy euthyroid volunteers received an oral dose of 80 μCi 125I in order to label endogenously produced thyroid hormones. Five days later 20 μCi 131I‐T4 were injected intravenously to serve as an internal reference source of T4 deiodination rate. The TSH response to an oral dose of 3 mg TRH given on two consecutive days was evaluated by radioimmunoassay and by determination of urinary 125/131I ratios.An increase of the urinary 125/131I ratio of the order of 200% (PP= 0.02) indicated an identical stimulation of the thyroid gland on both days. Following the first dose of TRH the serum TSH the serum TSH level rose by an average of 35% (P<0.05) whereas no change of TSH concentration could be detected on the second day of TRH administration. The findings indicate that mild TSH‐mediated stimulation of the thyroid gland may escape detection by radioimmunoassay of serum TSH concentration. Thus, the sole failure of serum TSH levels to rise by a measurable amount does not at present constitute proof for the autoregulatory nature of an observed change of thyroid function.