A patient has an elevated plasma thyroxine (T4) concentration, a low plasma TSH concentration, and her thyroid gland is smaller than normal. What is the most likely explanation for these findings?

Regulation of Thyroid hormone synthesis: Option A: Decreased TSH will lead to decreased T4 Option B: in Grave's disease there is formation of thyroid-stimulating immunoglobulin (TSI), also known as long-acting thyroid stimulator (LATS), which is not subject to negative-feedback inhibition by thyroid hormone. Thus, T4 will be increased that will cause negative feedback inhibition of TSH (i.e. TSH will be decreased). Due to continuous stimulation of thyroid gland by LATS there will glandular hyperophy or goitre. Option C: Antithyroid substances (PTU = propylthiouracil; MMI= Methimazole) Propylthiouracil will decrease T4 concentration. Option D: Thyroid extract means patient is taking exogenous T3 and T4 hence increased T4 Increased T4 causes feedback inhibition of TSH hence decreased TSH which causes hypoplasia of gland . This is known as Wolff Chaikoff effect .