Suppression of thyrotropin in the low-thyroxine state of severe nonthyroidal illness.

In a prospective study, we assessed the role of thyrotropin in the development of the low-thyroxine state that is associated with severe illness. We measured the serum thyrotropin and thyroid hormone concentrations longitudinally in 35 patients with hematopoietic cancer or aplastic anemia who were treated by bone-marrow transplantation. In 19 patients thyroxine declined sharply after bone-marrow transplantation and was associated with a reduction of the serum thyrotropin in the 17 patients tested, often to levels below the normal range. The serum triiodothyronine level, free thyroxine index, and free thyroxine level also declined in these patients. In the patients who recovered, clinical improvement was accompanied by the return of thyrotropin and thyroid hormone concentrations to their pretreatment ranges. These and related findings suggest that the low-thyroxine state of severe illness is the result of several events, one of which is failure of the normal negative-feedback control of the pituitary-thyroid axis due to illness-associated, decreased secretion of thyrotropin. The notion that such patients are "euthyroid" must be questioned, but the possible value of thyroid hormone-replacement therapy in these circumstances remains to be determined.