**Core Concept**
Thyroid hormones play a crucial role in regulating metabolism, growth, and development. They exert their effects by binding to specific receptors in the nucleus, which then modulate gene expression.

**Why the Correct Answer is Right**
Thyroid hormones bind to thyroid hormone receptors (THRs), a type of nuclear receptor that belongs to the steroid hormone receptor family. These receptors are encoded by two genes, THRA and THRB, and are found in various tissues, including the liver, kidneys, and brain. The thyroid hormone-receptor complex then acts as a transcription factor, regulating the expression of genes involved in metabolism, growth, and differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct type of receptor for thyroid hormones. Thyroid hormones do not bind to G-protein-coupled receptors (GPCRs) like many other hormones.
**Option B:** This option is incorrect because it refers to a different type of hormone receptor. Insulin receptors are a type of receptor tyrosine kinase, which are distinct from nuclear receptors like THRs.
**Option C:** This option is incorrect because it is too broad. While thyroid hormones do have effects on the cardiovascular system, this option does not accurately describe the specific type of receptor involved.

**Clinical Pearl / High-Yield Fact**
Thyroid hormone receptors are unique in that they can bind to both T3 and T4, but T3 is the more potent form. This is because T3 can bind to THRs with higher affinity than T4, which is converted to T3 in tissues.

**Correct Answer: D. Thyroid hormone receptors (THRs)**