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 due to other hereditary causes (e.g. Barakat syndrome (HDR syndrome) a genetic development disorder resulting in hypoparathyroidism, sensorineural deafness, and kidney disease)

Mechanism
The parathyroid glands are so named because they are usually located behind the thyroid gland in the neck. They arise during fetal development from structures known as the third and fourth pharyngeal pouch. The glands, usually four in number, contain the parathyroid chief cells that sense the level of calcium in the blood through the calcium-sensing receptor and secrete parathyroid hormone. Magnesium is required for PTH secretion. Under normal circumstances, the parathyroids secrete PTH to maintain a calcium level within normal limits, as calcium is required for adequate muscle and nerve function (including the autonomic nervous system). PTH acts on several organs to increase calcium levels. It increases calcium absorption in the bowel, while in the kidney it prevents calcium excretion and increases phosphate release and in bone it increases calcium through bone resorption. Diagnosis
Diagnosis is by measurement of calcium, serum albumin (for correction) and PTH in blood.
In patients where the likelihood of splenomegaly is high, the physical exam is not sufficiently sensitive to detect it; abdominal imaging is indicated in such patients.In cases of infectious mononucleosis splenomegaly is a common symptom and health care providers may consider using abdominal ultrasonography to get insight into a persons condition. However, because spleen size varies greatly, ultrasonography is not a valid technique for assessing spleen enlargement and should not be used in typical circumstances or to make routine decisions about fitness for playing sports. Causes
The most common causes of splenomegaly in developed countries are infectious mononucleosis, splenic infiltration with cancer cells from a hematological malignancy and portal hypertension (most commonly secondary to liver disease, and sarcoidosis). Splenomegaly may also come from bacterial infections, such as syphilis or an infection of the hearts inner lining (endocarditis). Splenomegaly also occurs in mammals parasitized by Cuterebra fontinella.The possible causes of moderate splenomegaly (spleen <1000 g) are many, and include:

The causes of massive splenomegaly (spleen >1000 g) are

chronic myelogenous leukemia
myelofibrosis
malaria
splenic marginal zone lymphoma

Pathophysiology
Splenomegaly can be classified based on its pathophysiologic mechanism:

Congestive, by pooled blood (e.g., portal hypertension)
Infiltrative, by invasion by cells foreign to the splenic environment (e.g., metastases, myeloid neoplasms, lipid storage diseases)
Immune, by an increase in immunologic activity and subsequent hyperplasia (e.g., endocarditis, sarcoidosis, rheumatoid arthritis)
Neoplastic, when resident immune cells originate a neoplasm (e.g., lymphoma). Diagnosis
Abdominal CT is the most accurate and spleen need to be larger 2-3 times to be palpable below the costal margin in physical examination that can help in diagnosis.