In this illustration, diagram A depicts the production of parathyroid hormone (PTH) by a tumor in the lung. The PTH hormone is functional, and acts on the normal target tissue, the bone, to mobilize calcium. Normally an increased blood calcium level would provide a negative feedback designed to stop the production of additional PTH. Due to the lack of functional receptors on the PTH-producing tumor, PTH production continues, and blood calcium levels remain high. The long-term effects of this process include loss of bone density, osteoporosis, increased tendency to form kidney stones, and development of muscle tetany from sustained elevated blood calcium levels.
Diagram B depicts the normal relationship between the parathyroid gland and calcium homeostasis, where calcium mobilized from the bone provides an effective negative feedback mechanism to the parathyroid gland to stop producing additional PTH.