Based on the results of some studies, thyroid hormones could also affect renin production and its release by the cells of the juxtaglomerular apparatus and serum renin activity.36, 37 Increased sympathetic outflow was thought to be a major cause of increased plasma renin activity in hyperthyroidism;38 however, increased sympathetic outflow has been observed in hypothyroidism.39 Adding to that, the removal of the thyroid gland 36 and methimazole‐induced hypothyroidism reduces serum renin activity.37 This evidence concerns the gene REN and hyperthyroidism.