The oral TRH stimulation test. A new diagnostic tool in "non-toxic" multinodular goitre.

We have developed a 28-hour TRH stimulation test based on the FT3I response to repeated oral TRH stimulation. The normal FT3I response in 15 controls was greater than or equal to 40 arb.U. Twenty-five euthyroid patients with multinodular goitre were studied. Fourteen had normal FT3I responses and normal basal levels of serum T4, FT4I, serum T3 and FT3I. Eleven patients had FT3I responses below 20 arb.U. Although the patients' individual basal values of serum T4, FT4I, serum T3 and FT3I were within the normal range, the median values of FT4I, serum T3 and FT3I were significantly elevated compared to the controls, suggesting that these patients were in a "pretoxic" condition. This assumption seems to be supported by the fact that 3 of these patients subsequently became frankly hyperthyroid, whereas all patients with normal FT3I response remained euthyroid. A comparison between the TSH response to i.v. TRH and the FT3I response to oral TRH showed concordance in 21 of the 25 patients studied.