Where does thyroid hormone activity primarily occur within the cell?

Thyroid hormones primarily exert their effects through intracellular receptors. These hormones, such as thyroxine (T4) and triiodothyronine (T3), are lipophilic and can easily pass through the cell membrane. Once inside the cell, they bind to specific receptors located in the cytoplasm or the nucleus.

After binding to these intracellular receptors, the hormone-receptor complex can then translocate into the nucleus, where it interacts with DNA to regulate gene expression. This mechanism is a key feature of steroid and thyroid hormones, where the ultimate physiological effects are mediated at the level of gene transcription.

In contrast, the other locations mentioned, such as the cell membrane, outside the nucleus, and in the endoplasmic reticulum, are not the primary sites for thyroid hormone activity. While some signaling may occur at the cell membrane for hydrophilic hormones, thyroid hormones operate mainly through their intracellular receptors to influence gene expression processes critical for metabolic regulation and development.