**Core Concept**
The onset of puberty is characterized by a series of physiological changes, primarily driven by hormonal fluctuations. The early hormonal sign of puberty is the increase in levels of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the pituitary gland to release gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

**Why the Correct Answer is Right**
The increase in GnRH secretion is the initial step in the pubertal cascade, leading to the development of secondary sexual characteristics. GnRH binds to its receptors in the hypothalamus, triggering a cascade of downstream effects, including the release of FSH and LH. These gonadotropins then stimulate the gonads (ovaries in females and testes in males) to produce sex hormones, such as estrogen and testosterone. The increase in estrogen levels, in particular, is responsible for the early signs of puberty in females, including breast development and pubic hair growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the early hormonal sign of puberty. While growth hormone plays a role in growth and development, it is not the primary hormone responsible for the early signs of puberty.
**Option B:** This option is incorrect because thyroid hormone is not directly involved in the early hormonal signs of puberty. While thyroid hormone is essential for growth and development, it is not the primary hormone responsible for the onset of puberty.
**Option C:** This option is incorrect because cortisol is a stress hormone that is not directly involved in the early hormonal signs of puberty. While cortisol plays a role in the body's response to stress, it is not the primary hormone responsible for the onset of puberty.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the onset of puberty is a complex process, and the timing and progression of pubertal changes can vary significantly between individuals. The early hormonal sign of puberty, the increase in GnRH levels, is a critical step in the pubertal cascade, and understanding this process is essential for diagnosing and managing disorders of puberty.

**Correct Answer:** A. Growth Hormone