SUMMARYThe thyroidal component of the response to thyrotrophin releasing hormone (TRH) has been studied by measurement of serum triiodothyronine (T3) and thyroxine (T4).In normal subjects a significant increase in serum T3 followed intravenous administration of TRH 200 μg. A less consistent increase was detected in serum T4 concentration.Oral TRH at a dose of 40 mg caused a significant rise in serum T4 in normal subjects. The increase in T4 at 24 hr was considered to provide a convenient index of pituitary and thyroidal response.Fourteen patients with pituitary disease, of whom twelve were euthyroid, were investigated by an oral TRH test. The majority of these patients retain a normal pituitary TSH reserve though a small group can be identified in whom it is diminished.A significant correlation was obtained between the T4 and TSH response to oral TRH. It is suggested that measurement of serum concentration of thyroid hormone is of value in the assessment of the pituitary response to TRH and the integrity of the pituitary‐thyroid ax