Distinguishing hypothyroxinemia due to euthyroid sick syndrome from pituitary insufficiency

Patients with severe nonthyroidal illness may have low serum levels of thyroid hormone and thyroid-stimulating hormone (TSH) indistinguishable from levels in patients with pituitary insufficiency. It is often difficult prospectively to rule out pituitary insufficiency in these patients. Our hypothesis was that patients sufficiently ill to have low free thyroxine index (FT4I) and TSH from nonthyroidal illness (euthyroid sick syndrome, or ESS) would have serum cortisol levels high enough to make pituitary insufficiency unlikely. Serum samples from all patients admitted to the Intensive Care Unit during 2 months were screened for low FT4I, and cortisol levels were measured on those samples. Five of five patients with a diagnosis of ESS had unequivocal elevations of serum cortisol (> 525 nmol/l), arguing against a diagnosis of pituitary insufficiency. Secondary hypothyroidism due to pituitary insufficiency can often be ruled out in patients with severe ESS by documenting appropriate elevated levels of serum cortisol.