The calcium that gives strength to your iguana's bones is in constant flux with the calcium circulating in the iguana's blood. Blood calcium is necessary to regulating its heartbeat, transmission of nerve impulses and muscle contractions. When blood calcium levels drop too low, calcium moves from the iguana's bones to its blood stream and vice versa. Calcium is also slowly lost from its body through its kidneys. If the iguana's diet does not supply sufficient new calcium, if the animal is not exposed to enough natural sunlight or if its diet is too low in vitamin D3, the animal will slowly withdraw calcium from its bones so that its blood calcium levels to not drop dangerously low. Calcium is what makes bones show up on x-rays. As less an less calcium is present in the bones, the pet can no longer support its weight, the bones bend or break and the image of your pet's bones on x-rays becomes fainter and fainter - as in the idealized images I drew below.