Lipophilic (hydrophobic) hormones circulate in the blood largely bound to plasma proteins.

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Lipophilic (hydrophobic) hormones such as steroids and thyroid hormones are insoluble in water and cannot travel freely in the bloodstream. Therefore, they bind to specific carrier proteins in the blood, which protect them from degradation and deliver them to target tissues. Only a small fraction of lipophilic hormones are unbound or free, which is what determines their biological activity.

Lipophilic (Hydrophobic) Hormones Circulate in the Blood Largely Bound to Plasma