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Cholesterol, which is hydrophobic, cannot simply travel in the bloodstream, and instead is enclosed in lipoprotein (LDL, HDL). Steroids, which are derived from cholesterol, are also hydrophobic (right?). So how do they travel in the bloodstream? If they are enclosed by anything, what is the molecular mechanism of their entering the target cell?

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Steroid hormones are transported in an inactive form bound to proteins in the bloodstream. Androgens and estrogen are bound to Sex hormone-binding globulin (SHBG), while glucocorticoids and progestins are bound to transcortin.

References:

  1. Transport of steroid hormones: binding of 21 endogenous steroids to both testosterone-binding globulin and corticosteroid-binding globulin in human plasma.
  2. Plasma protein-mediated transport of steroid and thyroid hormones.
  3. The Functions of Corticosteroid-Binding Globulin and Sex Hormone-Binding Globulin: Recent Advances
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  • $\begingroup$ They also bind to albumin. $\endgroup$ – pbond Nov 10 '15 at 18:55

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