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Exam questions · Biology · Hormones and Reproduction

Thyroxine and Negative Feedback (Higher tier)

  • 6 exam questions
  • 19 marks
  • 9 quick checks
  1. 1 State [1 mark]

    State what is meant by metabolic rate.

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    Model answer

    The speed of the chemical reactions in the cells of the body.

    Mark scheme

    • Rate of chemical reactions in the cells — 1 mark
  2. 2 Name [2 marks]

    Name the hormone that is released by the hypothalamus, and the hormone that is released by the pituitary gland, in the control of thyroxine.

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    Model answer

    The hypothalamus releases TRH, and the pituitary gland releases TSH.

    Mark scheme

    • TRH — 1 mark
    • TSH — 1 mark
  3. 3 Describe [3 marks]

    Describe what happens in the body when the level of thyroxine in the blood falls below normal.

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    Model answer

    The hypothalamus makes TRH, which stimulates the pituitary gland to release TSH. The TSH acts on the thyroid gland, which makes more thyroxine, so the level rises.

    Mark scheme

    • Hypothalamus makes TRH — 1 mark
    • Pituitary gland releases TSH — 1 mark
    • Thyroid gland makes more thyroxine — 1 mark
  4. 4 Explain [3 marks]

    Explain why this is an example of negative feedback.

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    Model answer

    The change, a low level of thyroxine, causes a response that increases thyroxine. When the level is normal, the thyroxine inhibits TRH and TSH, so that the response stops. The response reverses the change, which is negative feedback.

    Mark scheme

    • Response reverses the change — 1 mark
    • Normal level inhibits TRH and TSH — 1 mark
    • So the level stays steady — 1 mark
  5. 5 Explain [4 marks]

    A person has a thyroid gland that makes too much thyroxine. Explain the effects on the person, and on the levels of TRH and TSH.

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    Model answer

    The metabolic rate increases, so the person feels hot and restless and may lose mass. The high level of thyroxine inhibits the release of TRH and TSH, so the levels of TRH and TSH are low.

    Mark scheme

    • High metabolic rate — 1 mark
    • Feels hot, restless, loses mass — 1 mark
    • High thyroxine inhibits TRH and TSH — 1 mark
    • So their levels are low — 1 mark
  6. 6 Evaluate [6 marks]

    Some people in the past developed a goitre because they did not have enough iodine in their diet. Explain how the lack of iodine leads to a goitre, and why adding iodine to salt can prevent it.

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    Model answer

    The thyroid gland needs iodine to make thyroxine. Without it, the gland cannot make enough thyroxine, so the level in the blood is low. A low level does not inhibit the hypothalamus and the pituitary gland, so there is no negative feedback, and they keep releasing TRH and TSH. TSH stimulates the thyroid gland, and makes it grow larger, which is the goitre. If iodine is added to salt, people get enough to make thyroxine, so the level of thyroxine rises to normal, the negative feedback is restored, TSH falls, and the gland stays a normal size.

    Mark scheme

    • Iodine is needed to make thyroxine — 1 mark
    • Low level of thyroxine — 1 mark
    • No negative feedback, so TRH and TSH stay high — 1 mark
    • TSH makes the gland grow — 1 mark
    • Iodine in salt allows thyroxine to be made — 1 mark
    • Feedback restored, so TSH falls — 1 mark

Quick check

  1. 1

    Which gland makes thyroxine?

    1. APituitary
    2. BAdrenal
    3. CThyroid
    4. DPancreas
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    C: Thyroid

    The thyroid gland.

  2. 2

    Which hormone does the hypothalamus release in the control of thyroxine?

    1. ATRH
    2. BTSH
    3. CInsulin
    4. DAdrenaline
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    A: TRH

    TRH.

  3. 3

    What does TSH stimulate?

    1. AThe hypothalamus
    2. BThe liver
    3. CThe kidneys
    4. DThe thyroid gland
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    D: The thyroid gland

    TSH stimulates the thyroid gland to make thyroxine.

  4. 4

    What effect does a high level of thyroxine have on TRH release?

    1. AIt stimulates it
    2. BIt inhibits it
    3. CIt has no effect
    4. DIt destroys it
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    B: It inhibits it

    It inhibits it, which is negative feedback.

  5. 5

    What does thyroxine control?

    1. ABlood glucose
    2. BHeart rate only
    3. CThe metabolic rate
    4. DThe water content of the blood
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    C: The metabolic rate

    The metabolic rate.

  6. 6

    What would a person with too little thyroxine be likely to feel?

    1. ATired and cold
    2. BHot and restless
    3. CVery hungry
    4. DVery strong
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    A: Tired and cold

    Tired and cold, because the metabolic rate is low.

  7. 7

    What is negative feedback?

    1. AA response that makes a change bigger
    2. BA kind of hormone
    3. CA bad response
    4. DA response that reverses a change
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    D: A response that reverses a change

    A response that reverses a change.

  8. 8

    What element is needed to make thyroxine?

    1. AIron
    2. BIodine
    3. CCalcium
    4. DSodium
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    B: Iodine

    Iodine.

  9. 9

    In a person with a low level of thyroxine caused by a thyroid gland that does not work, which is likely to be found?

    1. ALow TSH
    2. BLow TRH and TSH
    3. CHigh TRH and TSH
    4. DNormal TRH and TSH
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    C: High TRH and TSH

    With no feedback from thyroxine, TRH and TSH levels are high.