A person has a normally functioning thyroid gland. Thyroxine is infused directly into the person's blood and the infusion is later stopped.
A laboratory method distinguishes infused thyroxine from thyroxine released by the person's thyroid gland. The total thyroxine concentration includes both sources.
| stage | infused thyroxine detectable | total thyroxine concentration compared with normal | TSH concentration compared with normal | concentration of thyroxine released by the thyroid gland compared with normal |
|---|---|---|---|---|
| before the infusion | no | normal | normal | normal |
| during the infusion, after the feedback response | yes | above normal | below normal | below normal |
| later, when infused thyroxine is no longer detectable | no | below normal | above normal | below normal |
Use the information in the table to explain the changes in TSH concentration and in the concentration of thyroxine released by the thyroid gland.
Suggest the next likely change in the concentration of thyroxine released by the thyroid gland.
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.