Receptors for thyroid hormones are present:
**Core Concept**
Thyroid hormones play a crucial role in regulating metabolism, growth, and development. The effects of thyroid hormones are mediated through specific receptors. These receptors are intracellular, suggesting that thyroid hormones can diffuse across cell membranes to interact with them.
**Why the Correct Answer is Right**
The receptors for thyroid hormones are indeed intracellular, specifically located in the **nucleus** of target cells. They belong to the nuclear receptor superfamily, which includes receptors for steroid hormones, vitamin D, and retinoic acid. Upon binding to thyroid hormones, these receptors regulate the expression of target genes by binding to specific DNA sequences.
**Why Each Wrong Option is Incorrect**
**Option A:** Mitochondria are the site of cellular respiration and energy production, not typically associated with thyroid hormone receptors.
**Option B:** The cell membrane is the site for many hormone receptors, but thyroid hormone receptors are unique in being intracellular.
**Option C:** The endoplasmic reticulum is involved in protein and lipid synthesis and is not the primary location for thyroid hormone receptors.
**Clinical Pearl / High-Yield Fact**
Remembering that thyroid hormone receptors are intracellular and nuclear helps in understanding how these hormones can have widespread effects on gene expression and cellular metabolism. This is crucial for diagnosing and managing thyroid disorders.
**Correct Answer:** D. Nucleus.