What type of receptors do thyroid hormones primarily act upon to exert their effects?

Thyroid hormones primarily exert their effects by acting upon nuclear receptors. These hormones, notably thyroxine (T4) and triiodothyronine (T3), are lipid-soluble and can easily cross cell membranes to bind to their corresponding nuclear receptors within the cell.

Once bound to these receptors, thyroid hormones engage in direct interaction with the DNA in the nucleus, leading to the regulation of gene expression. This mechanism allows for a broad range of physiological effects, including metabolic regulation, growth, and development. The action through nuclear receptors is distinct from other signaling pathways such as those involving G protein-coupled receptors or other kinases, which rely on secondary messengers and quicker cellular responses.

In summary, nuclear receptors are crucial for the action of thyroid hormones as they facilitate the transcriptional regulation of genes that underlie the diverse functions of these hormones in the body.