Hormones which act on intracellular receptors both initially in the cytoplasm and ultimately on nuclear receptors are categorized under:
**Core Concept**
Steroid hormones and thyroid hormones are lipid-soluble hormones that can diffuse through the cell membrane and bind to intracellular receptors, which are located both in the cytoplasm and the nucleus. This unique property allows them to exert their effects by altering gene expression.
**Why the Correct Answer is Right**
These hormones bind to their specific receptors in the cytoplasm, forming a hormone-receptor complex. This complex then translocates to the nucleus, where it binds to specific DNA sequences called hormone response elements, thereby regulating the transcription of target genes. This mechanism is distinct from that of steroid hormone receptors, which typically remain in the cytoplasm and do not translocate to the nucleus.
**Why Each Wrong Option is Incorrect**
**Option A:** Steroid hormones do not initially bind to intracellular receptors in the cytoplasm; instead, they typically bind to membrane-bound receptors.
**Option B:** Thyroid hormones do indeed act on intracellular receptors, but they are not the primary example of hormones that act on both cytoplasmic and nuclear receptors.
**Option C:** Catecholamines, such as epinephrine, act on membrane-bound receptors and do not exhibit the characteristics described in the question.
**Clinical Pearl / High-Yield Fact**
Remember that steroid hormones can be divided into two categories: those that act in the cytoplasm (e.g., aldosterone) and those that act in the nucleus (e.g., estrogen).
**Correct Answer: B. Thyroid hormones**