Hyperthyroidism is caused by an overactive thyroid gland - Edexcel - GCSE Biology Combined Science - Question 6 - 2020 - Paper 1

The correct answer is C: endocrine system. This is because the endocrine system is responsible for hormone production and regulation, which includes the function of the thyroid gland.

Negative feedback involving the thyroid gland regulates metabolic rate through the following steps:

1. Low Thyroxine Levels: When the levels of thyroxine are low, it stimulates the release of Thyrotropin-Releasing Hormone (TRH) from the hypothalamus.

2. TSH Release: TRH triggers the pituitary gland to release Thyroid-Stimulating Hormone (TSH).

3. Thyroxine Production: TSH stimulates the thyroid gland to produce more thyroxine, increasing its levels in the bloodstream.

4. Feedback Inhibition: As thyroxine levels rise, they inhibit the release of TRH and TSH, creating a negative feedback loop that stabilizes thyroid hormone levels and thus regulates metabolic rate.

Hormones play a crucial role in regulating the menstrual cycle through a series of interactions:

1. Follicle-Stimulating Hormone (FSH): At the beginning of the cycle, FSH is released from the pituitary gland and stimulates ovarian follicles to mature.

2. Estrogen Production: As follicles develop, they secrete estrogen, which prepares the uterine lining for potential implantation.

3. LH Surge: The increase in estrogen levels triggers a surge in Luteinizing Hormone (LH), leading to ovulation—the release of an egg from the ovary.

4. Progesterone Role: Following ovulation, the ruptured follicle becomes the corpus luteum, which secretes progesterone. This hormone maintains the uterine lining for a potential embryo.

5. Cycle Reset: If fertilization does not occur, progesterone and estrogen levels drop, leading to menstruation, which marks the beginning of a new cycle.