Fig. 15.32. 
Schematic representation of the
changes in the hypothalamo–pituitary–
thyroid axis in hypothalamic hypothyroidism. 
In this form of ‘tertiary’ hypothyroidism, the sixty-minute TSH response in the TRH test is equal to or greater than the twenty-minute response. TSH measured by RIA may also be normal or increased in some patients (see text).