Lesson 6 ·Class 10 · The Endocrine System · Flow Lab
Feedback Control of Hormone Levels
Too much, slow down; too little, speed up
Walk the TSH loop, the syllabus example of negative feedback.
→ → → → →
📉 Step 1 of 6 · Thyroxine falls below normal
The amount of thyroxine circulating in the blood drops below the normal level the body tries to maintain.
50%
TSH released by the pituitary 50%
Free play — step through the process at your own pace, and move the slider to see what changes. Switch to when you're ready to test yourself.
Negative feedback always commands the OPPOSITE: too much, slow down; too little, speed up. Almost every feedback loop in the body is of this negative kind. The rare positive example is uterine contraction at childbirth, where one contraction commands more contraction until delivery is complete.
What this lesson covers
What you do
- Write the four names in the loop in order: hypothalamus, TRH, anterior pituitary, TSH, thyroid, thyroxine.
- Explain why hormones are not stored in the body but excreted out.
- Say why uterine contraction at childbirth is positive rather than negative feedback.
Challenges to clear
- Step through the whole loop.
- Reach the stage where the loop shuts itself down.
- Drop thyroxine low enough to make the pituitary pour out TSH.
The task
Walk the TSH loop, the syllabus example of negative feedback.
What this lesson covers
- Thyroxine falls below normal — The amount of thyroxine circulating in the blood drops below the normal level the body tries to maintain.
- Hypothalamus sends TRH — The hypothalamus releases thyroid-releasing hormone, TRH, which stimulates the anterior pituitary.
- Pituitary sends TSH — The anterior pituitary secretes thyroid stimulating hormone, TSH, which travels in the blood to the thyroid gland.
- Thyroid secretes thyroxine — Stimulated by TSH, the thyroid gland pours thyroxine into the blood and the level begins to climb.
- Level rises above normal — The concentration of thyroxine in the blood now exceeds the normal level.
- Negative feedback switches it off — The excess thyroxine inhibits the secretion of TRH by the hypothalamus and of TSH by the anterior pituitary, so the thyroid slows down and the normal level is restored.
- Low thyroxine lifts the brake, so TRH and TSH pour out and the thyroid is driven harder.
- High thyroxine inhibits both TRH and TSH, so the thyroid is ordered to slow down.
Hint
Negative feedback always commands the OPPOSITE: too much, slow down; too little, speed up. Almost every feedback loop in the body is of this negative kind. The rare positive example is uterine contraction at childbirth, where one contraction commands more contraction until delivery is complete.