Steroid hormones (cortisol, progesterone, estradiol-17β, testosterone) and thyroid hormones (thyroxine, triiodothyronine) serve an essential function by regulating gene transcription. Cortisol is a steroid hormone that is produced by the adrenal gland. It is often called the "stress hormone" because it is secreted at higher levels in response to stress. Prolonged elevation of cortisol levels in the bloodstream can have negative effects on thyroid function, metabolism, and immune function. Measurement of cortisol in serum can be used to diagnose problems in the adrenal or pituitary gland, as well as stress-related disorders. Previous work has indicated that routine clinical measurements may significantly overestimate the cortisol levels present in serum, making accurate diagnosis of cortisol-related disorders difficult. Similar limitations have been observed in routine clinical assays for other hormones as well. Improving the accuracy of clinical assays for hormones will improve diagnoses and result in earlier treatments. The new reference methods and SRM 971 Hormones in Human Serum will help achieve this goal and will also provide higher-order reference systems for traceability.