Positive feedback is where a response to a stimulus causes an amplification in the response – during labour, for example. It is a much less common response than negative feedback which is used to maintain physiological and homeostatic control. On the other hand, most negative feedback mechanisms of a variable work with a receptor, control centre and effector, like this:
- The receptor detects change in the value of the variable
- The receptor passes the information to the control centre
- The control centre monitors the change in the variable
- If the change falls too far outside the normal (healthy) range of values, the control centre signals one or more effectors
- The effectors ‘take action’ and the variable returns to a level within the desired range or ‘set point’
Hormones and nerve signals stimulate changes which can be effected by further hormonal change or muscle action, for example.
A simple example of negative feedback is with body temperature, and responses – summarised in the table below – can be physiological and behavioural:
|RESPONSES WHEN TOO HOT||RESPONSES WHEN TOO COLD|
|Vasodilation allows heat to escape||Vasoconstriction takes heat further into body|
|Erector pili muscles relax (and hair lays down, allowing warmth to escape)||Erector pili muscles contract (and hair stands up, trapping air and heat)|
|Sweating increases (drawing away latent heat)||Sweating decreases|
|Shade and cool air sought||Warm areas sought|
|Clothes taken off||More clothes added|
Negative feedback in thyroid production
A more complex example of negative feedback in action is with thyroid production (needed for regulation of metabolism). The process is summarised here and explained in more detail below:
- The hypothalamus releases TRH onto pituitary thyrotrope cells
- The thyrotrope cells release TSH into the blood stream
- The TSH diffuses out of the blood stream into the thyroid gland
- The thyroid gland releases T4 (and T3) into the blood stream
- The T4 reaches the anterior pituitary and reduces the release of TSH
- The T4 reaches the hypothalamus and reduces the release of TRH
Thyroid production results from a multi-loop feedback system in the hypothalamic–pituitary–thyroid axis (also known as the HPT axis, thyroid homeostasis or thyrotropic feedback control).
A neuron in the paraventricular nucleus within the hypothalamus releases TRH (also known as thyrotropin releasing hormone or thyroliberin) into the hypothyseal portal vein; the TRH travels into the anterior pituitary.
The anterior pituitary contains thyrotrope cells which are part of the endocrine system: in response to the stimulation of TRH, they release another hormone, TSH (also known as thyroid stimulating hormone or thyrotropin), into the systemic circulation.
The TSH stimulates the thryroid to release T4 (thyroxine or tetraiodothyronine) and, to a lesser degree, T3 (triiodothyronine). Thyroid hormone (T4) exerts negative feedback control. That is, when there is enough T4 in the blood stream, the anterior pituitary and hypothalamus respond negatively by decreasing their production of TSH and TRH.
When things go wrong
Hypothyroidism and hyperthyroidism can be caused by a number of factors (see Q5 and ‘effects of stress’, below); in relation to negative feedback, they can be caused by problems occurring the pituitary or thyroid part of the loop.
When TSH is elevated, the thyroxine level should increase. If it doesn’t, this means there must be a problem in the thyroid (as the thyroid is not responding to the high levels of TSH by releasing thyroxine).
When thyroxine levels are low, TSH should be high. If it’s not, there must be a problem in hypothalamus or the pituitary (because lower levels of thyroxine should be stimulating an increase in TSH/TRH).
When TSH levels are elevated, TSH should drop. If it doesn’t, there must be a problem in pituitary (because thyroxine should be suppressing TSH).
When thyroxine levels are low but thyroid hormone levels are high, this indicates a problem in the thyroid (as it’s not responding to the decreased TSH and is still releasing high levels of thyroid hormone)
Effects of stress
Chronic stress leads to overproduction of the hormone cortisol. Cortisol and its precursor (CRH) can inhibit thyroid-stimulating hormone (TSH) and also inhibit conversion of the thyroid hormones T4 into T3.
Low T3 can lead to hypothyroid symptoms. However, simple nutrient and lifestyle modifications can help break this cycle and restart the normal cycle.
Now nutrition can help
Click here to see this post on nutrients for good thyroid help.
 The major portion of T3 is produced in peripheral organs, such as the liver, adipose tissue, glia and skeletal muscle; it is produced by deiodination from circulating T4. (Deiodination is controlled by numerous hormonal and nerve signals including TSH, vasopressin and catecholamines.)
Picture – creative commons