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Small Hydrophobic Ligands: Steroid Hormones

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A steroid hormone is a small, hydrophobic ligand built on a four-ring skeleton, able to diffuse across the plasma membrane to reach an internal receptor.

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Testosterone and estradiol must bind carrier proteins in blood, since their hydrophobic structure makes them poorly soluble alone.

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Do not conclude this steroid is water-soluble — the carrier protein, not its own polarity, keeps it in circulation.