It might have been anticipated that TRH would be useful as a diagnostic agent to differentiate between pituitary and hypothalamic causes of TSH deficiency. Indeed, many patients with disease of the hypothalamus which has caused TRH deficiency show responses to injected TRH, while others with pituitary disease fail to respond to TRH (see Fig. 1.12). There are, however, a sufficient number of cases not showing the classical or predicted response to prevent clear-cut differential diagnosis. In such cases, accurate radiological studies and other endocrine evaluations are required for diagnosis.
