The correct answer is 2. By a feedback loop.
Hormone levels in the body is regulated by negative feedback mechanism. In this mechanism, a stimulus stimulates the release of a substances. After the effect of that substance its further release is inhibited. This can be understood by an example, the anterior pituitary gland stimulate the thyroid gland to release thyroxin and on increasing the thyroxin in the blood, feedback the hypothalamus and anterior pituitary to inhibit further release of thyroxin from thyroid gland.
