Lipid derivatives, such as steroids, and thyroid hormones, which are amino acid derivatives. They are not soluble in water (hydrophobic), so must be carried in plasma bound to transporters. Only free hormones (that is, not bound to their transporter) enter cells.
Hydrophobic hormones are divided into two structural classes: lipid derivatives and amino acid derivatives. Lipid derivatives are derived from cholesterol (such as steroids) and fatty acids (such as prostaglandins). Amino acid derivatives include thyroid hormones, which are iodinated, so are not water-soluble. Insoluble hormones readily enter cells and bind to intracellular receptors to alter gene expression.
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