In addition to classifying hormones as steroidal or non-steroidal, hormones can also be classified by their solubility in water or fat and their transportation needs.
Amino acid and polypeptide hormones are generally hydrophilic (water-soluble) substances that travel freely in the blood. Amino acid and polypeptide hormones are repelled by areas of high lipid concentration such as membranes of the cell and nucleus and need a channel to bypass the phospholipid bilayer. Water-soluble hormones generally bind to extracellular targets on the plasma membrane.
Hormones such as estrogen, testosterone, progesterone, cortisol, and those produced in the thyroid are fat-soluble and repel water (hydrophobic). Fat-soluble hormones "like" lipid structures such as cell and nuclear membranes. Fat-soluble (water-insoluble) steroid hormones need special protein carriers for transportation throughout the body. Albumin is the most commonly used plasma protein for transporting lipid hormones.
If hormones were present in a mixture of oil and water, the amino acid-derived and polypeptide hormones would prefer to be in the water layer, and the steroid hormones would prefer to be in the oil layer.
