intracellular receptor
An intracellular receptor waits inside the cell rather than on its doorstep. Fat-soluble hormones — steroids, thyroid hormone, vitamin D — can slip straight through the oily cell membrane the way oil dissolves into oil. Once inside, they meet a receptor floating in the cytoplasm or already sitting in the nucleus.
These receptors bind their lipophilic hormone in the cell's interior, and the resulting hormone-receptor complex typically travels to the DNA and changes which genes are read. Because the action works through gene transcription and the making of new proteins, the response is comparatively slow — hours to days — but often long-lasting and powerful, reshaping how the cell behaves over time.
Intracellular receptors include the nuclear receptors for steroid and thyroid hormones. A caveat: although the textbook division is “water-soluble outside, fat-soluble inside,” some hormones have additional, faster membrane actions, so the clean split is a useful simplification rather than an absolute rule.
Cortisol diffuses into a liver cell, binds its glucocorticoid receptor in the cytoplasm, and the complex enters the nucleus to switch on genes for glucose-producing enzymes over the following hours.
Fat-soluble hormone enters the cell and acts on genes — slow but durable.
Lipophilic hormones travel in blood bound to carrier proteins because they cannot dissolve freely in watery plasma; only the small free fraction enters cells to reach intracellular receptors.