That steroid hormones are transported bound to carrier proteins in the bloodstream is a fundamental, well-established biological fact that requires no citation.
The claim that steroid hormones are transported bound to carrier proteins in the bloodstream is a matter of basic scientific education and everyday physiological observation (common knowledge). While papers such as [0] and [6] discuss the specific carrier proteins like albumin, SHBG, and CBG in detail, textbook scientific facts of this nature do not require citation and are classified as common knowledge.