Basal TSH values measured by an ultra-sensitive method (TSH-US) were compared with the response of TSH under TRH (delta TSH) in 178 patients. In all thyroid pathologies and in patients without thyroid or hypophyseal pathology, results are in good agreement, i.e., delta TSH is proportional to basal TSH-US (r = 0.93). In 92 patients with hypophyseal diseases, no correlation between delta TSH and basal TSH-US could be found.