American Board of Orthodontics (ABO) Practice Exam

The correct hormone that, along with fibroblast growth factor 23 (FGF23), decreases plasma phosphate is parathyroid hormone (PTH). PTH plays a critical role in regulating calcium and phosphate levels in the blood. This hormone is released by the parathyroid glands in response to low calcium levels in the blood, and it has a major influence on the kidneys, bones, and intestines.

When PTH is secreted, it acts on the kidneys to promote the excretion of phosphate, thus lowering its plasma concentration. Additionally, PTH stimulates the production of active vitamin D (calcitriol), which in turn increases intestinal absorption of calcium but also indirectly affects phosphate metabolism. While FGF23 primarily decreases circulating phosphate levels by inhibiting renal reabsorption of phosphate and suppressing vitamin D activation, PTH complements this action by directly enhancing phosphate excretion in the urine.

Other hormones such as calcitonin, insulin, or cortisol do not directly function in coordination with FGF23 to specifically regulate phosphate levels in the same context as PTH. Hence, PTH is essential in the calcium-phosphate balance in conjunction with FGF23, making it the correct answer to the question.