In normal thyroid metabolism, thyroid hormone production is tightly regulated by the hypothalamic-pituitary-thyroid axis. The pituitary gland releases TSH in response to thyroid releasing hormone (TRH) from the hypothalamus, which monitors T4 and T3 levels in the bloodstream. TSH binds to its receptor on the thyroid gland and stimulates production and release of T4 and T3. If T4 and T3 levels fluctuate outside of a normal range, TSH production is increased or lowered to compensate.
Thyroid Stimulating Immunoglobulin (TSI) mimics the action of TSH on the thyroid gland in that it causes T4 to be produced, but TSI is not regulated by the same negative feedback system. As a result TSI continues to stimulate the thyroid even after hormone levels are elevated, leading to an overabundance of hormones, and thus hyperthyroidism.