Secondary polycythemia is caused by either natural or artificial increases in the production of erythropoietin, hence an increased production of erythrocytes. In secondary polycythemia, 6 to 8 million and occasionally 9 million erythrocytes may occur per millimeter of blood. Secondary polycythemia resolves when the underlying cause is treated.

Secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia.

Conditions which may result in a physiologically appropriate polycythemia include:

- Altitude related - This physiologic polycythemia is a normal adaptation to living at high altitudes (see altitude sickness). Many athletes train at high altitude to take advantage of this effect — a legal form of blood doping. Some individuals believe athletes with primary polycythemia may have a competitive advantage due to greater stamina. However, this has yet to be proven due to the multifaceted complications associated with this condition.

- Hypoxic disease-associated - for example in cyanotic heart disease where blood oxygen levels are reduced significantly, may also occur as a result of hypoxic lung disease such as COPD and as a result of chronic obstructive sleep apnea.

- Iatrogenic - Secondary polycythemia can be induced directly by phlebotomy (blood letting) to withdraw some blood, concentrate the erythrocytes, and return them to the body.

- Genetic - Heritable causes of secondary polycythemia also exist and are associated with abnormalities in hemoglobin oxygen release. This includes patients who have a special form of hemoglobin known as Hb Chesapeake, which has a greater inherent affinity for oxygen than normal adult hemoglobin. This reduces oxygen delivery to the kidneys, causing increased erythropoietin production and a resultant polycythemia. Hemoglobin Kempsey also produces a similar clinical picture. These conditions are relatively uncommon.

Conditions where the secondary polycythemia is not as a result of physiologic adaptation and occurs irrespective of body needs include:

- Neoplasms - Renal-cell carcinoma or liver tumors, von Hippel-Lindau disease, and endocrine abnormalities including pheochromocytoma and adrenal adenoma with Cushing's syndrome.

- People whose testosterone levels are high because of the use of anabolic steroids, including athletes who abuse steroids, or people on testosterone replacement for hypogonadism or transgender hormone replacement therapy, as well as people who take erythropoietin, may develop secondary polycythemia.

The blood count typically shows decreased numbers of blood cells—including a decreased amount of circulating red blood cells, white blood cells, and platelets.

The bone marrow may show hemophagocytosis.

The liver function tests are usually elevated. A low level of the protein albumin in the blood is common.

The serum C reactive protein, erythrocyte sedimentation rate, and ferritin level are markedly elevated. In children, a ferritin above 10000 is very sensitive and specific for the diagnosis of HLH, however, the diagnostic utility for ferritin is less for adult HLH patients.

The serum fibrinogen level is usually low and the D-dimer level is elevated.

The sphingomyelinase is elevated.

Bone marrow biopsy shows histiocytosis.

Primary polycythemias are due to factors intrinsic to red cell precursors. Polycythemia vera (PCV), polycythemia rubra vera (PRV), or erythremia, occurs when excess red blood cells are produced as a result of an abnormality of the bone marrow. Often, excess white blood cells and platelets are also produced. PCV is classified as a myeloproliferative disease. Symptoms include headaches and vertigo, and signs on physical examination include an abnormally enlarged spleen and/or liver. In some cases, affected individuals may have associated conditions including high blood pressure or formation of blood clots. Transformation to acute leukemia is rare. Phlebotomy is the mainstay of treatment. A hallmark of polycythemia is an elevated hematocrit, with Hct > 55% seen in 83% of cases. A somatic (non-hereditary) mutation (V617F) in the "JAK2" gene is found in 95% of cases, though also present in other myeloproliferative disorders.

Primary familial polycythemia, also known as primary familial and congenital polycythemia (PFCP), exists as a benign hereditary condition, in contrast with the myeloproliferative changes associated with acquired PCV. In many families, PFCP is due to an autosomal dominant mutation in the "EPOR" erythropoietin receptor gene. PFCP can cause an increase of up to 50% in the oxygen-carrying capacity of the blood; skier Eero Mäntyranta had PFCP, which is considered to have given him a large advantage in endurance events.

The current (2008) diagnostic criteria for HLH are

1. A molecular diagnosis consistent with HLH. These include the identification of pathologic mutations of PRF1, UNC13D, or STX11.

OR

2. Fulfillment of five out of the eight criteria below:

- Fever (defined as a temperature >100.4 °F, >38 °C)

- Enlargement of the spleen

- Decreased blood cell counts affecting at least two of three lineages in the peripheral blood:

- Haemoglobin <9 g/100 ml (in infants <4 weeks: haemoglobin <10 g/100 ml) (anemia)

- Platelets <100×10/L (thrombocytopenia)

- Neutrophils <1×10/L (neutropenia

- High blood levels of triglycerides (fasting, greater than or equal to 265 mg/100 ml) and/or decreased amounts of fibrinogen in the blood (≤ 150 mg/100 ml)

- Ferritin ≥ 500 ng/ml

- Haemophagocytosis in the bone marrow, spleen or lymph nodes

- Low or absent natural killer cell activity

- Soluble CD25 (soluble IL-2 receptor) >2400 U/ml (or per local reference laboratory)

In addition, in the case of familial HLH, no evidence of malignancy should be apparent.

It should be noted that not all five out of eight criteria are required for diagnosis of HLH in adults, and a high index of suspicion is required for diagnosis as delays results in increased mortality. The diagnostic criteria were developed in pediatric populations and have not been validated for adult HLH patients. Attempts to improve diagnosis of HLH have included use of the HScore, which can be used to estimate an individual's risk of HLH.

Familial LPL deficiency should be considered in anyone with severe hypertriglyceridemia and the chylomicronemia syndrome. The absence of secondary causes of severe hypertriglyceridemia (like e.g. diabetes, alcohol, estrogen-, glucocorticoid-, antidepressant- or isotretinoin-therapy, certain antihypertensive agents, and paraproteinemic disorders) increases the possibility of LPL deficiency. In this instance besides LPL also other loss-of-function mutations in genes that regulate catabolism of triglyceride-rich lipoproteins (like e.g. ApoC2, ApoA5, LMF-1, GPIHBP-1 and GPD1) should also be considered

The diagnosis of familial lipoprotein lipase deficiency is finally confirmed by detection of either homozygous or compound heterozygous pathogenic gene variants in "LPL" with either low or absent lipoprotein lipase enzyme activity.

Lipid measurements

· Milky, lipemic plasma revealing severe hyperchylomicronemia;

· Severely elevated fasting plasma triglycerides (>2000 mg/dL);

LPL enzyme

· Low or absent LPL activity in post-heparin plasma;

· LPL mass level reduced or absent in post-heparin plasma;

Molecular genetic testing

The LPL gene is located on the short (p) arm of chromosome 8 at position 22. More than 220 mutations in the LPL gene have been found to cause familial lipoprotein lipase deficiency so far.

Definitive diagnosis requires LCAT gene analysis for mutation and functional activity. However, numerous lab tests may help with making a diagnosis such as complete blood count (CBC), urinalysis, blood chemistries, lipid panels, and plasma LCAT activity.

Fish-eye disease is characterized by abnormalities like visual impairment, plaques of fatty material, and dense opacification.

The U.S. Preventive Services Task Force in 2008 strongly recommends routine screening for men 35 years and older and women 45 years and older for lipid disorders and the treatment of abnormal lipids in people who are at increased risk of coronary heart disease. They also recommend routinely screening men aged 20 to 35 years and women aged 20 to 45 years if they have other risk factors for coronary heart disease. In 2016 they concluded that testing the general population under the age of 40 without symptoms is of unclear benefit.

In Canada, screening is recommended for men 40 and older and women 50 and older. In those with normal cholesterol levels, screening is recommended once every five years. Once people are on a statin further testing provides little benefit except to possibly determine compliance with treatment.

Myomatous erythrocytosis syndrome describes an excessive erythrocyte (red blood cells) production, occurring in about 0.5% of individuals affected by uterine leiomyomas (fibroids). This syndrome is believed to be caused by increased erythropoietin (EPO) production by the kidneys or by the leiomyomas themselves.

Testing the general population under the age of 40 without symptoms is of unclear benefit.

Osteomyelitis (bone infection), which is much more common than infantile cortical hyperostosis, must be excluded, since it requires urgent treatment. Other diagnoses that can mimic this disorder and need to be excluded include physical trauma, child abuse, Vitamin A excess, hyperphosphatemia, prostaglandin E1 and E2 administration, scurvy, infections (including syphilis), Ewing sarcoma, and metastatic neuroblastoma.

Renal failure is the major cause of morbidity and mortality in complete LCAT deficiency, while in partial deficiency (fish eye disease) major cause of morbidity is visual impairment due to corneal opacity. These patients have low HDL cholesterol but surprisingly premature atherosclerosis is not seen. However, there are some reported cases.

Lifestyle changes recommended for those with high cholesterol include: smoking cessation, limiting alcohol consumption, increasing physical activity, and maintaining a healthy weight.

Overweight or obese individuals can lower blood cholesterol by losing weight - on average a kilogram of weight loss can reduce LDL cholesterol by 0.8 mg/dl.

Most infants with infantile cortical hyperostosis are diagnosed by physical examination. X-rays can confirm the presence of bone changes and soft tissue swelling. Biopsy of the affected areas can confirm the presence of typical histopathological changes. No specific blood tests exist, but tests such as erythrocyte sedimentation rate (ESR) and alkaline phosphatase levels are often elevated. A complete blood count may show anemia (low red blood cell count) and leukocytosis (high white blood cell count). Other tests may be done to help exclude other diagnoses. Ultrasound imaging can help diagnose prenatal cases.

Treatment of LPLD has two different objectives: immediate prevention of pancreatitis attacks and long term reduction of cardiovascular disease risk. Treatment is mainly based on medical nutrition therapy to maintain plasma triglyceride concentration below 11,3 mmol/L (1000 mg/dL). Maintenance of triglyceride levels below 22,6 mmol/L (2000 mg/dL) prevents in general from recurrent abdominal pain.

Strict low fat diet and avoidance of simple carbohydrates

Restriction of dietary fat to not more than 20 g/day or 15% of the total energy intake is usually sufficient to reduce plasma triglyceride concentration, although many patients report that to be symptom free a limit of less than 10g/day is optimal. Simple carbohydrates should be avoided as well. Medium-chain triglycerides can be used for cooking, because they are absorbed into the portal vein without becoming incorporated into chylomicrons. Fat-soluble vitamins A, D, E, and K, and minerals should be supplemented in patients with recurrent pancreatitis since they often have deficiencies as a result of malabsorption of fat. However, the diet approach is difficult to sustain for many of the patients.

Lipid lowering drugs

Lipid-lowering agents such as fibrates and omega-3-fatty acids can be used to lower TG levels in LPLD, however those drugs are very often not effective enough to reach treatment goals in LPLD patients. Statins should be considered to lower elevated non-HDL-Cholesterol.

Additional measures are avoidance of agents known to increase endogenous triglyceride levels, such as alcohol, estrogens, diuretics, isotretinoin, anidepressants (e.g. sertraline) and b-adrenergic blocking agents.

Gene therapy

In 2012, the European Commission approved alipogene tiparvovec (Glybera), a gene therapy for adults diagnosed with familial LPLD (confirmed by genetic testing) and suffering from severe or multiple pancreatitis attacks despite dietary fat restrictions. It was the first gene therapy to receive marketing authorization in Europe; it was priced at about $1 million per treatment, and as of 2016, only one person had been treated with it.

As most cases of FHH are asymptomatic and benign, the diagnosis of FHH is less likely to be made.

Typically, diagnosis is made in the pursuit of uncovering the etiology of hypercalcemia.

Calcium levels are often in the high normal range or slightly elevated.

Commonly, the parathyroid hormone level is checked and may be slightly elevated or also on the high normal end.

Normally, high calcium should cause low PTH and so this level of PTH is inappropriately high due to the decreased sensitivity of the parathyroid to calcium.

This may be mistaken for primary hyperparathyroidism.

However, evaluation of urine calcium level will reveal a low level of urine calcium.

This too is inappropriate as high serum calcium should result in high urine calcium.

If urine calcium is not checked, this may lead to parathyroidectomy for presumed primary hyperparathyroidism.

Additionally as the name implies, there may be a family history of benign hypercalcemia.

Ultimately, diagnosis of familial hypocalciuric hypercalcemia is made — as the name implies — by the combination of low urine calcium and high serum calcium.

No treatment is generally required, as bone demineralisation and kidney stones are relatively uncommon in the condition.

Familial dysalbuminemic hyperthyroxinemia is a type of hyperthyroxinemia associated with mutations in the human serum albumin gene.

The term was introduced in 1982.

Both conditions are treated with fibrate drugs, which act on the peroxisome proliferator-activated receptors (PPARs), specifically PPARα, to decrease free fatty acid production. Statin drugs, especially the synthetic statins (atorvastatin and rosuvastatin), can decrease LDL levels by increasing hepatic reuptake of LDL due to increased LDL-receptor expression.

Apolipoprotein B deficiency (also known as "Familial defective apolipoprotein B-100") is an autosomal dominant disorder resulting from a missense mutation which reduces the affinity of apoB-100 for the low-density lipoprotein receptor (LDL Receptor) . This causes impairments in LDL catabolism, resulting in increased levels of low-density lipoprotein in the blood. The clinical manifestations are similar to diseases produced by mutations of the LDL receptor, such as familial hypercholesterolemia. Treatment may include, niacin or statin or ezetimibe.

It is also known as "normotriglyceridemic hypobetalipoproteinemia".

In 2010, the case of a man with unexplained erythrocytosis and perinephric fluid collection as main features was described in the Case Records of the Massachusetts General Hospital. As a consequence two strikingly similar cases were identified and a review of the literature revealed three more patients with similar characteristics.

As of December 2014, a total of 9 patients worldwide with the TEMPI syndrome have been identified (D.B.Sykes, Personal Communication).

In terms of their therapy:

- Untreated: 2 patients

- Velcade alone: 5 patients

- Immediate autologous transplant: 1 patient

- Velcade followed by Velcade/Lenalidomide followed by autologous transplant: 1 patient

Familial dysbetalipoproteinemia or type III hyperlipoproteinemia (also known as remnant hyperlipidemia, "remnant hyperlipoproteinaemia", "broad beta disease" and "remnant removal disease") is a condition characterized by increased total cholesterol and triglyceride levels, and decreased HDL levels.

Clinical features along with the familial tendency may be enough to make a diagnosis. Genetic testing may also be used.

Canakinumab has been approved for treatment of HIDS and has shown to be effective. The immunosuppressant drugs etanercept and anakinra have also shown to be effective. Statin drugs might decrease the level of mevalonate and are presently being investigated. A recent single case report highlighted bisphosphonates as a potential therapeutic option.

Prenatal screening is not typically done for FHM, however it may be performed if requested. As penetrance is high, individuals found to carry mutations should be expected to develop signs of FHM at some point in life.

Familial dysautonomia is inherited in an autosomal recessive pattern, which means 2 copies of the gene in each cell are altered. If both parents are shown to be carriers by genetic testing, there is a 25% chance that the child will produce FD. Prenatal diagnosis for pregnancies at increased risk for FD by amniocentesis (for 14–17 weeks) or chorionic villus sampling (for 10–11 weeks) is possible.