Why does the thyroid have negative feedback

Regulatory mechanisms

The control of the function of the endocrine system can be described by control loops. The feedback on the success of a control process, also called "feedback", is based on the principles of cybernetics. The feedback is linked to the feedback, i.e. the processing of this information.
The secretion of a hormone is usually influenced by several interlocking control loops and also by internal and external (disruptive) factors. This relationship is shown in Figure 1: "Hormonal control loop". The stability guaranteed by the control mechanisms is not final or static, but dynamic. For example, the setpoint of a control loop can change in the sense of an adjustment or for physiological reasons.

Chart 1:

Hormonal control loop using the example of the human hypothalamus-pituitary axis (similar for many hormones);

peripheral hormonal glands are, for example, thyroid, kidney, liver, pancreas, etc .;

Success cells / organs are muscles, secretion cells (e.g. parietal cells in the stomach), mammary glands and others. m.

Graphic 2:

Localization of the superordinate hormonal glands hypothalamus and pituitary gland

The hypothalamus forms the release hormones ("releasing factors") due to hormonal or neural stimuli. These cause glandotropic hormones to be released in the anterior pituitary gland, which then later reach peripheral hormonal glands via the blood.

There they cause the glandular effector hormones to be released, which now again reach their target cells with the corresponding receptors via the bloodstream. Table 1 contains some of the hormones that occur in this control loop and are important for the human organism and their respective target glands.

Table 1:Hormones of the hypothalamus and pituitary gland and the affected organs


For example, TRH (fourth line in the table) induces the secretion of TSH, which increases the synthesis and secretion of thyroid hormone in the thyroid gland. Thyroid hormones in turn reduce the release of TSH in the anterior pituitary gland, which closes the circle. Such Feedback occurs either neural or hormonal and is usually negative feedback. As can be seen in Figure 1, the signal to inhibit the further release of hormones must be passed on directly to the hormone-secreting cells. It can be the released hormone itself (autocrine effect, see Figure 2a), or another hormone slows down the further secretion of hormones in the primary secreting glandular cell (see Figure 2b).
This process is schematized as follows:
 2 a)
Graphics 2a and 2b:
Negative feedback (greatly simplified control loop)
2 B)