Assessment of the clinically significant TSH response to TRH in patients with nodular goitre

Twenty-five patients with nodular goitre who had thyroid hormone levels within normal ranges and an absent thyrotropin (TSH) response to TSH releasing hormone (TRH) as measured by a conventional radioimmunoassay with a lower detection limit of 0.6 mU/l were studied. Based on these data, and the clinical evaluation patients were divided into a hyperthyroid group (n = 12) and a euthyroid group (n = 13). The samples from the TRH test were reanalyzed by an immunoradiometric TSH assay with a detection limit of 0.05 mU/l. Basal serum TSH showed a considerable overlap between the two groups, but values above 0.10 mU/l were always associated with euthyroidism. Using this level of discrimination 76% of the patients were correctly classified. A TSH response to TRH of 0.10 mU/l provided a better discrimination allowing a correct diagnosis in 92% of the patients. It is concluded that serum TSH as measured by a sensitive assay is suitable as a first line test in patients with nodular goitre. However, patients with basal serum TSH levels below 0.10 mU/l need further investigation with a TRH-test. A TSH response to TRH above 0.10 mU/l seems to secure euthyroidism, whereas lower responses almost always are associated with hyperthyroidism.