 Dear students, in this topic we shall discuss the mechanism of action of lipid soluble hormones in detail. First, we shall see how the lipid soluble hormones are transported through the blood to their target tissues. The lipid soluble steroid and thyroid hormones are carried through blood by forming complexes with some carrier proteins. These carrier proteins are necessary because you know that blood is an aqueous medium in which lipid soluble substances cannot dissolve. So they can be transported in undissolved form, but if they are transported in undissolved form, they will bind to any lipids which are present in circulation. So the binding of these hormones to carrier protein ensures their delivery to the target tissues. Now once they reach their target cells or tissues, they dissociate themselves from their carrier proteins. Being lipid soluble, they can easily enter into the cell by dissolving in the plasma membrane or inside the cell. These hormones have cytoplasmic receptors. By binding with these cytoplasmic receptors, hormones form the receptor hormone complex. These complexes then move into the nucleus. The all the receptors that bind any lipid soluble hormone have a highly conserved DNA binding domain. These receptors have a DNA binding site or domain which does not have a hormone, the receptor is an inhibitory protein linked or attached to this domain which keeps this site inactive and that is why the receptor is inactive state. But when the hormone comes and binds with the receptor, the inhibitory protein is removed. As a result, the DNA binding domain of the receptor is exposed and in this state receptor is said to be activated. So the DNA binding domain binds with the regulatory sequences of DNA. As a result, this binding causes the induction of transcription of specific genes which result in the synthesis of specific proteins. These students because the lipid soluble hormones act directly on the DNA and stimulate or inhibit the production of certain proteins, their effect persists for a long time which may be for hours to days. In comparison, the effects of lipid insoluble hormones are short lived and they may last for only few minutes or maximum to few hours.