Metabolic Bone Disease (MBD) is a blanket term that is used to refer to several disease syndromes related to problems with calcium homeostasis or metabolism. The cause of disease may be a lack of available calcium in the diet or it may be that for any one of a number of reasons the reptile is unable to use the calcium in the diet.

Calcium is required for more than just strong bones; it is necessary for the proper function of nerves and muscles. Calcium levels in the body are controlled within tight limits by complex hormone interactions. Bones act as a reservoir of calcium when there isn't enough available in the diet.

The absorption and metabolism of calcium within the body is very complex, and varies somewhat between species of reptiles. Many aspects of calcium regulation by the body remain to be determined. One simple explanation is that when ultraviolet (uv) rays from the sun contact the skin, they activate vitamin D, which is necessary for the absorption of dietary calcium from the digestive tract. In the case of reptiles, these biochemical reactions can occur only within the animal's Preferred Optimum Temperature Zone (POTZ). See Husbandry.

The syndromes associated with MBD arise from a breakdown in this chain of events. The cause of MBD is a husbandry problem. The diet may be low in calcium or it may contain an excess of other nutrients which prevent the absorption of calcium. The ultraviolet light source may be inadequate or the temperature at which the animal is kept may be incorrect.

Some species are more or less sensitive to inadequacies at a given point in this chain. The disease manifests more quickly and severely in young and growing animals.

Signs commonly associated with MBD can be divided into two categories:

Signs affecting the skeleton – These include swollen limbs, fractured or broken limbs (often the reptile just drags the affected limb), a fractured backbone or tail, soft shells in tortoises and turtles, a swollen, soft or pliable lower jaw and a general failure to grow. One of the most common signs of MBD, particularly in lizards such as the Green Iguana, is the inability to raise the entire body from the ground.

Signs related to low blood calcium – These include muscle twitching, seizures, paralysis, egg binding, difficulty passing waste, gastrointestinal blockages and changes in behaviour, such as lethargy or aggression.

The life threatening signs are often preceded by more subtle physical and behavioural changes. These include a lack of energy or reluctance to posture and display in a normal manner; it is crucial to know what to expect of a normal, healthy individual of your species of reptile. Calcium is necessary for the movement of the smooth muscles of the intestinal tract, and so early signs of a problem with calcium metabolism include bloating, decreased frequency of defecation and difficulty in passing stool or constipation.

In the case of the Green Iguana, it is common to see small muscle twitches or tremors in the days and weeks before seizures begin. These fine, repetitive, involuntary movements, usually affect the toes and tail, and may worsen as the animal is stressed by observation or handling, or when he tries to move.