In a healthy thyroid, the pituitary signals the thyroid gland in response the amount of T3 and T4 circulating in the body. When thyroid hormone levels are high the pituitary will signal the thyroid to reduce the production of thyroid hormone and when levels are low the pituitary will signal to increase production. The steps for this process if listed below.
Step 1: Pituitary gland located in the brain detects a dip in thyroid hormone circulating in the body and releases TSH (thyroid stimulating hormone) into the bloodstream
Step 2: TSH travels to the thyroid where it signals the thyroid to release more T4
Step 3: T4 is released from the thyroid gland and either enters the tissues (free T4) or becomes inactive by attaching to proteins (bound T4). This is determined by how much thyroid is needed.
Step 4: Some of the free T4 is converted into T3 in the liver and enters the bloodstream to reach tissue cells. T3 has a much more substantial impact on the tissues than T4
(For this reason, even if your T4 and TSH are normal, but your T3 conversion is low, you may still have hypothyroid symptoms)
Step 5: The body sends signals back to the pituitary gland. If the amount of T3 and T4 are high the pituitary will stop sending TSH signals to the thyroid, if the T3 and T4 are low, the pituitary will send more TSH to the thyroid.