A negative feedback loop regulates the levels of thyroid hormones.
When circulating T3andT4 levels drop too low, the hypothalamus increases release of thyrotropin-releasing hormone (TRH), which acts on the anterior pituitary gland to stimulate the release of thyroid stimulating hormone (TSH), causing the thyroid gland to release more T4 and T3.
Disruption to this mechanism results in hypothyroidism or hyperthyroidism.