**Question:** Which one of the following hormonal levels is decreased after intracranial irradiation?

**Core Concept:** Intracranial irradiation refers to the use of ionizing radiation to treat brain tumors or other neurological disorders. This treatment can lead to damage to the brain tissue and secondary effects on the endocrine system.

**Why the Correct Answer is Right:** After intracranial irradiation, the pituitary gland can be affected, leading to a decrease in the production and release of various hormones. The pituitary gland is a small endocrine organ located in the brain, and it plays a crucial role in regulating the endocrine system. It contains two main parts: the anterior pituitary and the posterior pituitary.

**Why Each Wrong Option is Incorrect:**

A. Decreased cortisol levels after intracranial irradiation is incorrect because cortisol is primarily regulated by the hypothalamus and the adrenal glands, not the pituitary gland.

B. Decreased growth hormone levels are not directly caused by intracranial irradiation. Although growth hormone is regulated by the hypothalamus and anterior pituitary, the hypothalamus and pituitary are not directly targeted by intracranial irradiation.

C. Decreased thyroid-stimulating hormone (TSH) levels after intracranial irradiation are incorrect because TSH is primarily regulated by the hypothalamus and the pituitary, but the hypothalamus is more vulnerable to irradiation than the pituitary, leading to decreased TSH production.

D. Decreased adrenocorticotropic hormone (ACTH) levels after intracranial irradiation is incorrect because ACTH is regulated by the hypothalamus and the anterior pituitary gland. The hypothalamus is more susceptible to irradiation, making the pituitary less affected.

**Correct Answer:** C. Decreased thyroid-stimulating hormone (TSH) levels after intracranial irradiation.

Intracranial irradiation can damage the hypothalamus, which is more vulnerable to radiation than the pituitary gland. Consequently, the hypothalamus's ability to secrete thyrotropin-releasing hormone (TRH) is impaired, leading to a decrease in TSH production. This cascades down to affect thyroid hormone production and resulting in hypothyroidism.

**Why Each Wrong Option is Incorrect:**

A. Cortisol levels are regulated by the hypothalamus and the adrenal glands, not the pituitary gland. Therefore, cortisol levels are not directly affected by intracranial irradiation.

B. Growth hormone is regulated by the hypothalamus and anterior pituitary. However, the hypothalamus is more susceptible to irradiation than the pituitary gland, making growth hormone levels less affected by intracranial irradiation.

D. ACTH is regulated by the hypothalamus and the anterior pituitary gland. As mentioned before, the hypothalamus is more susceptible to irradiation, which means the pituitary gland is less affected. Therefore, ACTH levels are not directly affected by intracranial irradiation.

**Clinical Pearls:**

In clinical practice, hypothy