OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How does a negative feedback system regulate thyroxine levels?

1. By increasing levels of TSH when thyroxine is high

2. By inhibiting TSH secretion when thyroxine is high

3. By stimulating more thyroxine production

4. By promoting glucose utilization

The correct answer is: By inhibiting TSH secretion when thyroxine is high

A negative feedback system is crucial in maintaining homeostasis within the body. In the context of thyroxine regulation, when thyroxine levels in the blood are high, the body needs to prevent excessive production of this hormone, which is primarily produced by the thyroid gland. The correct response identifies that the system works by inhibiting the secretion of thyroid-stimulating hormone (TSH). When thyroxine levels rise above normal, the hypothalamus detects this change and reduces the release of thyrotropin-releasing hormone (TRH). In response, the pituitary gland decreases the production of TSH. Since TSH stimulates the thyroid gland to produce more thyroxine, lower levels of TSH will result in reduced thyroid activity, thereby helping to bring thyroxine levels back to a normal range. This feedback loop ensures that hormone levels remain balanced, preventing both excess and deficiency. The other choices do not accurately describe the mechanism of negative feedback in this context. For instance, increasing TSH levels when thyroxine is high would actually exacerbate the situation by prompting more thyroxine production, which contradicts the primary function of a negative feedback system. Similarly, stimulating more thyroxine production does not align with the goal of regulating