THE RESPONSE OF THE THYROID GLAND TO THYROTROPIC HORMONE AS AN AID IN THE DIFFERENTIAL DIAGNOSIS OF PRIMARY AND SECONDARY HYPOTHYROIDISM*†

Abstract

THE differentiation of hypothyroidism of pituitary origin from that due to primary impairment of the thyroid gland may be very difficult. Proper treatment is dependent upon correct distinction of the two conditions. The problem is met whenever hypometabolism is encountered along with evidence of diminished function of other endocrine glands, such as the adrenal cortex or the gonads. One must first decide whether the hypometabolism is a result of hypothyroidism; for in many conditions, such as anorexia nervosa, severe hypometabolism may exist when there is ample laboratory evidence of a normally functioning thyroid gland. The usual laboratory data on thyroid function do not suffice to distinguish between primary thyroid failure and pituitary failure. The trends are the same in both, and elevated serum cholesterol and axillary hair loss also may prevail in both. Diminished function of other target glands of the pituitary favors a diagnosis of hypothyroidism of pituitary origin. However, there may be diminished function of these glands in primary myxedema also (1): for instance, in a thyroidectomized rat the weight of the adrenal glands diminishes and the cortex shows changes in cellular pattern compatible with hypofunction. The clinical situation is further complicated by the existence of a bi-glandular disease (thyroid fibrosis accompanied by adrenal atrophy) recently observed by one of us (2). Thus, there are three conditions in which hypothyroidism is a feature of pluriglandular disease: i) pituitary insufficiency, ii) severe hypothyroidism causing hypoadrenocorticism, and iii) bi-glandular disease of the thyroid and adrenal glands with a normal pituitary gland.