<prompt> Which of the following hormones regulates blood levels of 1,25-OH-cholecalciferol positively:ThyroxineParathormoneInsulinCalcitonin  <response>: PTH - PTH acts in a positive feed forward manner in regulating vitamin D in response to S. Ca At Ca2 concentration below 9-10 mg 100 ml PTH promotes conversion of 25 -OH-cholecalciferol to 1,25 di OH cholecalciferol in the kidneys CALCITONIN - Peptide hormone secreted by thyroid gland - parafollicular cells C. cells - Tends to decrease serum Ca level that is effect opposite to PTH INSULIN - Causes intracellular movement of plasma glucose S. glucose 