12) In Graves' disease, the thyroid gland keeps secreting its hormones because abnormal _?_ won't let the hypothalamus and pituitary hormones stop it.

In Graves' disease, the thyroid gland keeps secreting its hormones because abnormal antibodies won't let the hypothalamus and pituitary hormones stop it. These antibodies are known as thyroid-stimulating immunoglobulins (TSIs). They attach to thyroid cells and mimic thyroid-stimulating hormone (TSH), tricking the thyroid into producing more hormones than the body needs.

Here are the steps:

1. The immune system produces abnormal antibodies known as thyroid-stimulating immunoglobulins (TSIs).
2. These TSIs attach to thyroid cells.
3. They mimic the action of thyroid-stimulating hormone (TSH), which is normally responsible for regulating the production of thyroid hormones.
4. The presence of TSIs tricks the thyroid gland into thinking that more thyroid hormones are needed.
5. As a result, the thyroid gland keeps producing and secreting thyroid hormones, even when the levels are already high.
6. The hypothalamus and pituitary hormones, which normally regulate the production of thyroid hormones, are unable to stop this overproduction because of the presence of the TSIs.