Final answer:
The presence of autoantibodies agonistic to T4 receptors disrupts the negative feedback loop leading to an overproduction of thyroid hormones and a decrease in TSH levels.
Step-by-step explanation:
If autoantibodies are agonistic to the T4 receptors, there is an increase in production of thyroid hormones due to the antibodies binding to the TSH receptors on the thyroid gland. Instead of the usual regulation of thyroid hormones through the negative feedback loop, these autoantibodies mimic thyroid-stimulating hormone (TSH) and continually stimulate the thyroid gland to release T3 and T4. This leads to an overproduction of the thyroid hormones T4 (thyroxine) and often T3 (triiodothyronine), which increases the metabolic rate of the body. As T3 and T4 levels rise in the blood, they should normally inhibit the release of TSH from the anterior pituitary; however, because the autoantibodies are continuously stimulating the thyroid gland, this feedback inhibition is overridden, resulting in low levels of TSH despite high levels of T4.