Hypercalcemia is suspected to occur in approximately 1 in 500 adults in the general adult population. Like hypocalcemia, hypercalcemia can be non-severe and present with no symptoms, or it may be severe, with life-threatening symptoms. Hypercalcemia is most commonly caused by hyperparathyroidism and by malignancy, and less commonly by vitamin D intoxication, familial hypocalciuric hypercalcemia and by sarcoidosis. Hyperparathyroidism occurs most commonly in postmenopausal women. Hyperparathyroidism can be caused by a tumor, or adenoma, in the parathyroid gland or by increased levels of parathyroid hormone due to hypocalcemia. Approximately 10% of cancer sufferers experience hypercalcemia due to malignancy. Hypercalcemia occurs most commonly in breast cancer, lymphoma, prostate cancer, thyroid cancer, lung cancer, myeloma, and colon cancer. It may be caused by secretion of parathyroid hormone-related peptide by the tumor (which has the same action as parathyroid hormone), or may be a result of direct invasion of the bone, causing calcium release.

Symptoms of hypercalcemia include anorexia, nausea, vomiting, constipation, abdominal pain, lethargy, depression, confusion, polyuria, polydipsia and generalized aches and pains.

Hypocalcemia is common and can occur unnoticed with no symptoms or, in severe cases, can have dramatic symptoms and be life-threatening. Hypocalcemia can be parathyroid related or vitamin D related. Parathyroid related hypocalcemia includes post-surgical hypoparathyroidism, inherited hypoparathyroidism, pseudohypoparathyroidism, and pseudo-pseudohypoparathyroidism. Post-surgical hypoparathyroidism is the most common form, and can be temporary (due to suppression of tissue after removal of a malfunctioning gland) or permanent, if all parathyroid tissue has been removed. Inherited hypoparathyroidism is rare and is due to a mutation in the calcium sensing receptor. Pseudohypoparathyroidism is maternally inherited and is categorized by hypocalcemia and hyperphosphatemia. Finally, pseudo-pseudohypoparathyroidism is paternally inherited. Patients display normal parathyroid hormone action in the kidney, but exhibit altered parathyroid hormone action in the bone.

Vitamin D related hypocalcemia may be associated with a lack of vitamin D in the diet, a lack of sufficient UV exposure, or disturbances in renal function. Low vitamin D in the body can lead to a lack of calcium absorption and secondary hyperparathyroidism (hypocalcemia and raised parathyroid hormone). Symptoms of hypocalcemia include numbness in fingers and toes, muscle cramps, irritability, impaired mental capacity and muscle twitching.

The neuromuscular symptoms of hypercalcemia are caused by a negative bathmotropic effect due to the increased interaction of calcium with sodium channels. Since calcium blocks sodium channels and inhibits depolarization of nerve and muscle fibers, increased calcium raises the threshold for depolarization. This results in diminished deep tendon reflexes (hyporeflexia), and skeletal muscle weakness. There is a general mnemonic for remembering the effects of hypercalcaemia: "Stones, Bones, Groans, Thrones and Psychiatric Overtones"

- Stones (renal or biliary) (see calculus)

- Bones (bone pain)

- Groans (abdominal pain, nausea and vomiting)

- Thrones (polyuria) resulting in dehydration

- Psychiatric overtones (Depression 30–40%, anxiety, cognitive dysfunction, insomnia, coma)

Other symptoms include cardiac arrhythmias (especially in those taking digoxin), fatigue, nausea, vomiting (emesis), anorexia, abdominal pain, constipation, & paralytic ileus. If renal impairment occurs as a result, manifestations can include polyuria, nocturia, and polydipsia. Psychiatric manifestation can include emotional instability, confusion, delirium, psychosis, & stupor. Limbus sign seen in eye due to hypercalcemia.

Hypercalcemia can result in an increase in heart rate and a positive inotropic effect (increase in contractility).

Symptoms are more common at high calcium blood values (12.0 mg/dL or 3 mmol/l). Severe hypercalcaemia (above 15–16 mg/dL or 3.75–4 mmol/l) is considered a medical emergency: at these levels, coma and cardiac arrest can result. The high levels of calcium ions decrease the neuron membrane permeability to sodium ions, thus decreasing excitability, which leads to hypotonicity of smooth and striated muscle. This explains the fatigue, muscle weakness, low tone and sluggish reflexes in muscle groups. The sluggish nerves also explain drowsiness, confusion, hallucinations, stupor and / or coma. In the gut this causes constipation. Hypocalcaemia causes the opposite by the same mechanism.

The neuromuscular symptoms of hypocalcemia are caused by a positive bathmotropic effect due to the decreased interaction of calcium with sodium channels. Since calcium blocks sodium channels and inhibits depolarization of nerve and muscle fibers,reduced calcium lowers the threshold for depolarization. The symptoms can be recalled by the mnemonic "CATs go numb" - convulsions, arrhythmias, tetany, and numbness in the hands and feet and around the mouth.

Hypocalcaemia, also spelled hypocalcemia, is low calcium levels in the blood serum. The normal range is 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L) with levels less than 2.1 mmol/L defined as hypocalcemia. Mildly low levels that develop slowly often have no symptoms. Otherwise symptoms may include numbness, muscle spasms, seizures, confusion, or cardiac arrest.

Common causes include hypoparathyroidism and vitamin D deficiency. Others causes include kidney failure, pancreatitis, calcium channel blocker overdose, rhabdomyolysis, tumor lysis syndrome, and medications such as bisphosphonates. Diagnosis should generally be confirmed with a corrected calcium or ionized calcium level. Specific changes may be seen on an electrocardiogram (ECG).

Initial treatment for severe disease is with intravenous calcium chloride and possibly magnesium sulfate. Other treatments may include vitamin D, magnesium, and calcium supplements. If due to hypoparathyroidism, hydrochlorothiazide, phosphate binders, and a low salt diet may also be recommended. About 18% of people who are in hospital have hypocalcemia.

Symptoms depend on whether the hyperparathyroidism is the result of parathyroid overactivity or secondary.

In primary hyperparathyroidism about 75% of people have no symptoms. The problem is often picked up during blood work for other reasons via a raised calcium. Many other people only have non-specific symptoms. Symptoms directly due to hypercalcemia are relatively rare, being more common in patients with malignant hypercalcemia. If present, common manifestations of hypercalcemia include weakness and fatigue, depression, bone pain, muscle soreness (myalgias), decreased appetite, feelings of nausea and vomiting, constipation, polyuria, polydipsia, cognitive impairment, kidney stones (See Foot Note) and osteoporosis. A history of acquired racquet nails (brachyonychia) may be indicative of bone resorption. Parathyroid adenomas are very rarely detectable on clinical examination. Surgical removal of a parathyroid tumor eliminates the symptoms in most patients.

In secondary hyperparathyroidism the parathyroid gland is behaving normally; clinical problems are due to bone resorption and manifest as bone syndromes such as rickets, osteomalacia and renal osteodystrophy.

Primary hyperparathyroidism and malignancy account for about 90% of cases of hypercalcaemia.

Bone and joint pain are common, as are limb deformities. The elevated PTH has also pleiotropic effects on the blood, immune system, and neurological system.

Secondary hyperparathyroidism (SHPT) refers to the excessive secretion of parathyroid hormone (PTH) by the parathyroid glands in response to hypocalcemia (low blood calcium levels) and associated hyperplasia of the glands. This disorder is especially seen in patients with chronic kidney failure. It is often—although not consistently—abbreviated as SHPT in medical literature.

The main symptoms of hypoparathyroidism are the result of the low blood calcium level, which interferes with normal muscle contraction and nerve conduction. As a result, people with hypoparathyroidism can experience paresthesia, an unpleasant tingling sensation around the mouth and in the hands and feet, as well as muscle cramps and severe spasms known as "tetany" that affect the hands and feet. Many also report a number of subjective symptoms such as fatigue, headaches, bone pain and insomnia. Crampy abdominal pain may occur. Physical examination of someone with hypocalcemia may show tetany, but it is also possible to provoke tetany of the facial muscles by tapping on the facial nerve (a phenomenon known as Chvostek's sign) or by using the cuff of a sphygmomanometer to temporarily obstruct the blood flow to the arm (a phenomenon known as Trousseau's sign of latent tetany).

A number of medical emergencies can arise in people with low calcium levels. These are seizures, severe irregularities in the normal heart beat, as well as spasm of the upper part of the airways or the smaller airways known as the bronchi (both potentially causing respiratory failure).

Hyperparathyroidism is an increased parathyroid hormone (PTH) levels in the blood. This occurs either from the parathyroid glands inappropriately making too much PTH (primary hyperparathyroidism) or other events triggering increased production by the parathyroid glands (secondary hyperparathyroidism). Most people with primary disease have no symptoms at the time of diagnosis. In those with symptoms the most common is kidney stones with other potential symptoms including weakness, depression, bone pains, confusion, and increased urination. Both types increase the risk of weak bones.

Primary hyperparathyroidism in 80% of cases is due to a single benign tumor known as a parathyroid adenoma with most of the rest of the cases due to a multiple benign tumors. Rarely it may be due to parathyroid cancer. Secondary hyperparathyroidism typically occurs due to vitamin D deficiency, chronic kidney disease, or other causes of low blood calcium. Diagnosis of primary disease is by finding a high blood calcium and high PTH levels.

Primary hyperparathyroidism may be cured by removing the adenoma or overactive parathyroid glands. In those without symptoms, mildly increased blood calcium levels, normal kidneys, and normal bone density monitoring may be all that is required. The medication cinacalcet may also be used to decrease PTH levels. In those with very high blood calcium levels treatment may include large amounts of intravenous normal saline. Low vitamin D levels should be corrected.

Primary hyperparathyroidism is the most common form. In the developed world between one and four per thousand people are affected. It occurs three times more often in women than men and is typically diagnosed between the ages of 50 and 60. The disease was first described in the 1700s and in the late 1800s was determined to be related to the parathyroid. Surgery as a treatment was first carried out in 1925.

The signs and symptoms of primary hyperparathyroidism are those of hypercalcemia. They are classically summarized by "stones, bones, abdominal groans, thrones and psychiatric overtones".

- "Stones" refers to kidney stones, nephrocalcinosis, and diabetes insipidus (polyuria and polydipsia). These can ultimately lead to renal failure.

- "Bones" refers to bone-related complications. The classic bone disease in hyperparathyroidism is osteitis fibrosa cystica, which results in pain and sometimes pathological fractures. Other bone diseases associated with hyperparathyroidism are osteoporosis, osteomalacia, and arthritis.

- "Abdominal groans" refers to gastrointestinal symptoms of constipation, indigestion, nausea and vomiting. Hypercalcemia can lead to peptic ulcers and acute pancreatitis. The peptic ulcers can be an effect of increased gastric acid secretion by hypercalcemia.

- "Thrones" refers to polyuria and constipation

- "Psychiatric overtones" refers to effects on the central nervous system. Symptoms include lethargy, fatigue, depression, memory loss, psychosis, ataxia, delirium, and coma.

Left ventricular hypertrophy may also be seen.

Other signs include proximal muscle weakness, itching, and band keratopathy of the eyes.

When subjected to formal research, symptoms of depression, pain, and gastric dysfunction seem to correlate with mild cases of hypercalcemia.

Signs and symptoms include ectopic calcification, secondary hyperparathyroidism, and renal osteodystrophy. Abnormalities in phosphate metabolism such as hyperphosphatemia are included in the definition of the new chronic kidney disease-mineral and bone disorder (CKD-MBD).

Hypoparathyroidism can have the following causes:

- Removal of, or trauma to, the parathyroid glands due to thyroid surgery (thyroidectomy), parathyroid surgery (parathyroidectomy) or other surgical interventions in the central part of the neck (such as operations on the larynx and/or pharynx) is a recognized cause. It is the most common cause of hypoparathyroidism. Although surgeons generally make attempts to spare normal parathyroid glands at surgery, inadvertent injury to the glands or their blood supply is still common. When this happens, the parathyroids may cease functioning. This is usually temporary but occasionally long term (permanent).

- Kenny-Caffey Syndrome

- Autoimmune invasion and destruction is the most common non-surgical cause. It can occur as part of autoimmune polyendocrine syndromes.

- Hemochromatosis can lead to iron accumulation and consequent dysfunction of a number of endocrine organs, including the parathyroids.

- Absence or dysfunction of the parathyroid glands is one of the components of chromosome 22q11 microdeletion syndrome (other names: DiGeorge syndrome, Schprintzen syndrome, velocardiofacial syndrome).

- Magnesium deficiency

- A defect in the calcium receptor leads to a rare congenital form of the disease

- Idiopathic (of unknown cause), occasionally familial (e.g. Barakat syndrome (HDR syndrome) a genetic development disorder resulting in hypoparathyroidism, sensorineural deafness and renal disease)

In contrast with primary hyperparathyroidism in adults, primary hyperparathyroidism in pediatric patients is considered a rare endocrinopathy. Pediatric primary hyperparathyroidism can be distinguished by its more severe manifestations, in contrast to the less intense manifestations in adult primary hyperparathyroidism. Multiple endocrine neoplasia is more likely to be associated with childhood and adolescent primary hyperparathyroidism. The fundamental skeletal radiologic manifestation include diffuse osteopenia, pathologic fractures and the coexistence of resorption and sclerosis at numerous sites. Skeletal lesions can be specifically bilateral, symmetric and multifocal, exhibiting different types of bone resorption. Pathologic fractures of the femoral neck and spine can potentially initiate serious complications. Because pediatric primary hyperparathyroidism is frequently associated with pathologic fractures it can be misdiagnosed as osteogenesis imperfecta. Pediatric patients with primary hyperparathyroidism are best remedied by parathyroidectomy. Early diagnosis of pediatric primary hyperparathyroidism is all-important to minimize disease complications and start off timely and relevant treatment.

Hyperphosphatemia is an electrolyte disturbance in which there is an abnormally elevated level of phosphate in the blood. Often, calcium levels are lowered (hypocalcemia) due to precipitation of phosphate with the calcium in tissues. Average phosphorus levels should be between 0.81 mmol/litre and 1.45 mmol/litre.

Metastatic calcification is deposition of calcium salts in otherwise normal tissue, because of elevated serum levels of calcium, which can occur because of deranged metabolism as well as increased absorption or decreased excretion of calcium and related minerals, as seen in hyperparathyroidism.

In contrast, dystrophic calcification is caused by abnormalities or degeneration of tissues resulting in mineral deposition, though blood levels of calcium remain normal. These differences in pathology also mean that metastatic calcification is often found in many tissues throughout a person or animal, whereas dystrophic calcification is localized.

Metastatic calcification can occur widely throughout the body but principally affects the interstitial tissues of the vasculature, kidneys, lungs, and gastric mucosa. For the latter three, acid secretions or rapid changes in pH levels contribute to the formation of salts.

The major symptoms of OFC are bone pain or tenderness, bone fractures, and skeletal deformities such as bowing of the bones. The underlying hyperparathyroidism may cause kidney stones, nausea, constipation, fatigue and weakness. X-rays may indicate thin bones, fractures, bowing, and cysts. Fractures are most commonly localized in the arms, legs, or spine.

The addition of weight loss, appetite loss, vomiting, polyuria, and polydipsia to the aforementioned symptoms may indicate that OFC is the result of parathyroid carcinoma. Parathyroid carcinoma, an uncommon cancer of the parathyroid glands, is generally indicated by serum calcium levels higher than usual, even in comparison to the high serum calcium levels that OFC generally presents with. Symptoms are also often more severe. Generally, the presence of a palpable neck mass is also indicative of the cancer, occurring in approximately 50% of sufferers, but virtually nonexistent in individuals with OFC with a different origin.

Characteristic symptoms are:

- Sudden penetrating pain in the legs, lower back or abdomen

- Confusion, psychosis, slurred speech

- Severe lethargy

- Convulsions

- Fever

- Hyperkalemia (elevated potassium level in the blood)

- Hypercalcemia (elevated calcium level in the blood): the cause of hypercalcemia is a combination of increased calcium input into the extracellular space and reduced calcium removal by the kidney, this last caused by decreased glomerular filtration and increased tubular calcium reabsorption. Both renal factors are secondary to volume depletion and, in fact, improve rapidly during rehydration with saline infusion.

- Hypoglycemia (reduced level of blood glucose)

- Hyponatremia (low sodium level in the blood)

- Hypotension (low blood pressure)

- Hypothyroid (low T4 level)

- Severe vomiting and diarrhea, resulting in dehydration

- Syncope (loss of consciousness and ability to stand)

Osteitis fibrosa cystica is defined as the classic skeletal manifestation of advanced hyperparathyroidism. Under the ICD-10 classification system, established by the World Health Organization, OFC is listed under category E21.0, primary hyperparathyroidism.

Adrenal crisis is caused by a deficiency of cortisol resulting from Addison's disease, congenital adrenal hyperplasia (CAH), corticosteroid biosynthetic enzyme defects or pituitary disorders (such as Sheehan's syndrome, pituitary adenoma, hypopituitarism (inactive or underactive pituitary) causing failure to activate the adrenal glands.

Classical signs of uremia are: progressive weakness and easy fatigue, loss of appetite due to nausea and vomiting, muscle atrophy, tremors, abnormal mental function, frequent shallow respiration and metabolic acidosis. Without intervention via dialysis or kidney transplant, uremia due to renal failure will progress and cause stupor, coma and death. Because uremia is mostly a consequence of kidney failure, its signs and symptoms often occur concomitantly with other signs and symptoms of kidney failure. Below is a table showing more of the principal signs and symptoms of uremia.

Glomerular filtration rate (GFR) measures the amount of plasma being filtered through the kidneys. As the GFR decreases, the prognosis worsens. Some of the effects can be reversed with dialysis. See below for a chart on GFR rates and their effects.

People on dialysis acquire what is known as "residual syndrome". Residual syndrome is a non-life-threatening disease which is displayed as toxic effects causing many of the same signs and symptoms that uremia displays. There are several hypotheses why residual syndrome is present. They are: (1) the accumulation of large molecular weight solutes that are poorly dialyzed (e.g. β-Microglobulin), (2) the accumulation of protein-bound small molecular weight solutes that are poorly dialyzed (e.g. "p"-cresyl sulfate and indoxyl sulfate), (3) accumulation of dialyzable solutes that are incompletely removed (e.g. sequestered solutes like phosphate in cells or insufficient elimination of other more toxic solutes), (4) indirect phenomena such as carbamylation of proteins, tissue calcification, or a toxic effect of hormone imbalance (e.g. Parathyroid hormone) and (5) the toxic effect by dialysis itself (e.g. removal of unknown important vitamins or minerals). Dialysis increases life span but patients may have more limited function. They gain physical limitations which include impairment of balance, walking speed and sensory functions. They also retain cognitive impairment such as impairment in attention, memory and performance of higher-order tasks. Patients have been maintained longer than three decades on dialysis, but unfortunately average mortality rates and hospitalizations are high. Also patient rehabilitation and quality of life is poor.

Abnormal heart rhythms and asystole are possible complications of hypermagnesemia related to the heart. Magnesium acts as a physiologic calcium blocker, which results in electrical conduction abnormalities within the heart.

Clinical consequences related to serum concentration:

- 4.0 mEq/l decreased reflexes

- >5.0 mEq/l Prolonged atrioventricular conduction

- >10.0 mEq/l Complete heart block

- >13.0 mEq/l Cardiac arrest

Note that the therapeutic range for the prevention of the pre-eclampsic uterine contractions is: 4.0-7.0 mEq/L. As per Lu and Nightingale, serum Mg concentrations associated with maternal toxicity (also neonate depression - hypotonia and low Apgar scores) are:

- 7.0-10.0 mEq/L - loss of patellar reflex

- 10.0-13.0 mEq/L - respiratory depression

- 15.0-25.0 mEq/L - altered atrioventricular conduction and (further) complete heart block

- >25.0 mEq/L - cardiac arrest

Renal osteodystrophy may exhibit no symptoms; if it does show symptoms, they include:

- Bone pain

- Joint pain

- Bone deformation

- Bone fracture

- The broader concept of chronic kidney disease-mineral and bone disorder (CKD-MBD) is not only associated with fractures but also with cardiovascular calcification, poor quality of life and increased morbidity and mortality in CKD patients (the so-called bone-vascular axis). These clinical consequences are acquiring such an importance that scientific working groups (such as the ERA-EDTA CKD-MBD Working Group) or international initiatives are trying to promote research in the field including basic, translational and clinical research.