An increase in the bloods level of a thyroid gland hormone decreases release stimulating hormone. what does this mechanism illustrate?

The mechanism illustrated here is a negative feedback loop.

Negative feedback loops are common in hormone regulation. In this case, the negative feedback loop helps to maintain homeostasis of the thyroid hormones in the blood. When the level of thyroid hormones increases, TRH and TSH release decreases thus inhibiting further synthesis of THs This prevents the excessive production of hormones.