9 (a) Hyperthyroidism is caused by an overactive thyroid gland - Edexcel - GCSE Biology - Question 9 - 2020 - Paper 1

C endocrine system. The thyroid gland is classified as part of the endocrine system, which regulates various hormonal functions, unlike the other options which pertain to different bodily systems.

Negative feedback is a crucial mechanism that maintains homeostasis in the body. In the case of the thyroid gland, when levels of thyroxine increase in the bloodstream, they inhibit the release of Thyroid Stimulating Hormone (TSH) from the pituitary gland. This, in turn, reduces further production of thyroxine from the thyroid. Conversely, if thyroxine levels drop, there is less inhibition on TSH secretion, leading to an increase in thyroxine production until balance is restored. This regulatory loop ensures that the metabolic rate remains stable and responsive to the body’s needs.

The menstrual cycle is regulated by a complex interplay of hormones, including estrogen, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). At the start of the cycle, FSH stimulates the growth of ovarian follicles. As follicles mature, they release estrogen, which promotes the thickening of the uterine lining. A surge in LH triggers ovulation, releasing an egg. After ovulation, the remaining follicle transforms into the corpus luteum, which secretes progesterone to prepare the uterus for potential implantation. If fertilization does not occur, progesterone levels drop, leading to the shedding of the uterine lining during menstruation. This hormonal regulation ensures the proper timing of ovulation and menstruation.