From the Research
The correct answer is A. Binds to nuclear receptors. Group 1 lipophilic hormones, which include steroid hormones and thyroid hormones, are characterized by their ability to cross cell membranes due to their lipid solubility. Once inside the cell, these hormones bind to nuclear receptors, which then act as transcription factors to regulate gene expression, as described in the study by 1. This is in contrast to the other options which are incorrect:
- Group 1 lipophilic hormones typically have low free concentrations in plasma because they are extensively bound to transport proteins (contradicting option B).
- When bound to transport proteins, these hormones are protected from metabolism, making them difficult to metabolize in this state (contradicting option C).
- Due to their extensive protein binding and resulting protection from metabolism, these hormones generally have long half-lives in plasma, not short ones (contradicting option D). The nuclear receptor binding mechanism is fundamental to how these hormones exert their physiological effects through genomic pathways, making option A the true statement, as supported by studies such as 2, 3, and 4, with the most recent and highest quality study being 1.
The mechanism of action of these hormones is further supported by the study 5, which describes the DNA recognition by nuclear receptors and how they bind to specific DNA sequences, called hormone-response elements (HREs), to regulate gene expression.
Overall, the evidence suggests that group 1 lipophilic hormones bind to nuclear receptors, which is essential for their physiological effects, and this is the correct answer.