A scientist uses a system containing tissues from the hypothalamus, pituitary gland and thyroid gland. At the start, the thyroxine concentration in the surrounding fluid is 13.0 pmol/L, within the normal range of 12 to 14 pmol/L. The thyroid tissue releases extra thyroxine. Hormones in the fluid are continually removed or broken down at a constant rate, and no other changes are made to the system. Samples of the fluid are taken at intervals.
The results are shown in the table.
| Time after extra thyroxine is released / hours | Thyroxine concentration / pmol/L | TSH concentration / mIU/L |
|---|---|---|
| 0 | 13.0 | 5.0 |
| 2 | 19.0 | 2.5 |
| 8 | 14.0 | 4.5 |
Explain how the results support the conclusion that negative feedback is occurring. Predict how the concentration of TRH changes between 0 and 2 hours.
89 exam-style questions on Edexcel GCSE Biology 7.1 Hormones and coordination, covering 7.1.1 Endocrine glands and hormone transport, 7.1.2 Adrenalin and fight or flight, and 7.1.3 Thyroxine and negative feedback. Each one has a worked solution and a mark scheme showing where the marks go.