PTH and secondary hyperparathyroidism: The authors concluded that in the FX rats (1) osteopenia was not caused by intestinal Ca mal-absorption, VD, Ca deficiency, or secondary hyperparathyroidism; (2) osteopenia might be related to PTH-independent urinary hyperexcretion of P, followed by a rise of serum 1, 25(OH)2D3; and (3) the existence of endocrine axis among gastrin, calcitonin, and PTH could not be substantiated.