Aim for FiveBiology

Conceptual analysis

Iodine and thyroid hormone feedback

  • Unit 4
  • 4 points
  • About 10 minutes

You can use a calculator on this question, just like on exam day.

A short question about a biological system that gets disrupted. You describe and explain the normal process, then predict what the disruption causes and justify that prediction. On the exam: Question 4 of 6 in Section II; about 10 minutes suggested.

The question

In humans, a region of the brain called the hypothalamus releases the hormone TRH, which signals the pituitary gland to release the hormone TSH into the blood. TSH binds to receptors on thyroid gland cells and stimulates them to make thyroid hormones, which contain iodine. When thyroid hormone levels in the blood rise, thyroid hormones inhibit the release of TRH and TSH. A person's diet does not contain enough iodine.

Suggested time: 10 minutes

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Part (a)

1 point

Describe negative feedback in a biological system.

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Part (b)

1 point

TSH travels throughout the body in the blood. Explain why TSH affects thyroid cells but not most other cells in the body.

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Part (c)

1 point

Predict the effect of the iodine-deficient diet on the level of TSH in the person's blood.

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Part (d)

1 point

Justify your prediction in part (c).

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