Psoriasis has been associated with obesity and several other cardiovascular and metabolic disturbances. The incidence of diabetes is 27% higher in people affected by psoriasis than in those without the condition. Severe psoriasis may be even more strongly associated with the development of diabetes than mild psoriasis. Younger people with psoriasis may also be at increased risk for developing diabetes. Individuals with psoriasis or psoriatic arthritis have a slightly higher risk of heart disease and heart attacks when compared to the general population. Cardiovascular disease risk appeared to be correlated with the severity of psoriasis and its duration. There is no strong evidence to suggest that psoriasis is associated with an increased risk of death from cardiovascular events. Methotrexate may provide a degree of protection for the heart.

The odds of having hypertension are 1.58 times higher in people with psoriasis than those without the condition; these odds are even higher with severe cases of psoriasis. A similar association was noted in people who have psoriatic arthritis—the odds of having hypertension were found to be 2.07 times greater when compared to odds of the general population. The link between psoriasis and hypertension is not currently understood. Mechanisms hypothesized to be involved in this relationship include the following: dysregulation of the renin-angiotensin system, elevated levels of endothelin 1 in the blood, and increased oxidative stress. The incidence of the heart rhythm abnormality atrial fibrillation is 1.31 times higher in people with mild psoriasis and 1.63 times higher in people with severe psoriasis. There may be a slightly increased risk of stroke associated with psoriasis, especially in severe cases. Treating high levels of cholesterol with statins has been associated with decreased psoriasis severity, as measured by PASI score, and has also been associated with improvements in other cardiovascular disease risk factors such as markers of inflammation. These cardioprotective effects are attributed to ability of statins to improve blood lipid profile and because of their anti-inflammatory effects. Statin use in those with psoriasis and hyperlipidemia was associated with decreased levels of high-sensitivity C-reactive protein and TNFα as well as decreased activity of the immune protein LFA-1. Compared to individuals without psoriasis, those affected by psoriasis are more likely to satisfy the criteria for metabolic syndrome.

The rates of Crohn's disease and ulcerative colitis are increased when compared with the general population, by a factor of 3.8 and 7.5 respectively. People with psoriasis also have a higher risk of celiac disease. Few studies have evaluated the association of multiple sclerosis with psoriasis, and the relationship has been questioned. Psoriasis has been associated with a 16% increase in overall relative risk for non-skin cancer. People with psoriasis have a 52% increased risk cancers of the lung and bronchus, a 205% increase in the risk of developing cancers of the upper gastrointestinal tract, a 31% increase in the risk of developing cancers of the urinary tract, a 90% increase in the risk of developing liver cancer, and a 46% increase in the risk of developing pancreatic cancer. The risk for development of non-melanoma skin cancers is also increased. Psoriasis increases the risk of developing squamous cell carcinoma of the skin by 431% and increases the risk of basal cell carcinoma by 100%. There is no increased risk of melanoma associated with psoriasis.

Guttate psoriasis accounts for approximately 2% of psoriasis cases.

Although there are likely to be multiple genetic factors and environmental triggers, mutations causing defects in the IL-36RN, CARD14 and AP1S3 genes have been shown to cause GPP .

Genetic and environmental factors can influence the predilection for guttate psoriasis. Human leukocyte antigens, especially those in the HLA-C group are associated with the skin disorder. Beta-hemolytic streptococci infection is the major contributing environmental factor. The typical route of infection is the upper respiratory system. Rarely it is also caused by a skin infection surrounding the anus (perianal streptococcal dermatitis).

Eosinophilic folliculitis associated with HIV infection typically affects individuals with advanced HIV and low T helper cell counts. It affects both men and women as well as children with HIV and is found throughout the world.

EF may also affect individuals with hematologic disease such as leukemia and lymphoma. It may also affect otherwise normal infants in a self-limited form. HIV-negative individuals can also develop EF — this is more common in Japan.

Most cases of generalized pustular psoriasis present in patients with existing or prior psoriasis conditions. However, there are many cases of GPP that arise without a history of psoriasis.

The Department of Dermatology of the University of São Paulo proposed a classification for these two conditions. Pso+ represents patients with a personal history of psoriasis and pso- represents patients with no history of psoriasis. They also identified a common factor among patients in each group: In the pso+ group, the most common precipitating factor is corticosteroid withdrawal. In the pso- group, the most common precipitating factor is infection.

In a large portion of cases, the disease is brought on by some triggering factor. Through research and observation, many of these factors have been identified. The following table, from an article in Cutis, lists a few factors that have been observed as influential in the onset of GPP.

Source: “Table II”, “Pustular Psoriasis” Farber and Nall, 1993

Pustular psoriasis is classified into two major forms: localized and generalized pustular psoriasis. Within these two categories there are several variants:

The term pustular psoriasis is used for a heterogeneous group of diseases that share pustular skin characteristics.

The classification of exfoliative dermatitis into Wilson-Brocq (chronic relapsing), Hebra or pityriasis rubra (progressive), and Savill (self-limited) types may have had historical value, but it currently lacks pathophysiologic or clinical utility.

The exact causes of dyshidrosis are unknown. In 2013, a randomized, double-blind, placebo-controlled cross-over study by the University Medical Center Groningen reported that dyshydrosis outbreaks on the hands increased significantly among those allergic to house dust mites, following inhalation of house dust mite allergen.

Food allergens may be involved in certain cases. Cases studies have implicated a wide range of foods including tuna, tomato, pineapple, chocolate, coffee, and spices among others. A number of studies have implicated balsam of Peru.

Id reaction and irritant contact dermatitis are possible causes.

About 1 in 2,000 people are affected in Sweden. Males and females appear to be affected equally.

A 2003 survey of diseases of the foot in 16 European countries found onychomycosis to be the most frequent fungal foot infection and estimates its prevalence at 27%. Prevalence was observed to increase with age. In Canada, the prevalence was estimated to be 6.48%. Onychomycosis affects approximately one-third of diabetics and is 56% more frequent in people suffering from psoriasis.

Erythroderma is generalized exfoliative dermatitis, which involves 90% or more of the patient's skin. The most common cause of erythroderma is exacerbation of an underlying skin disease, such as psoriasis, contact dermatitis, seborrheic dermatitis, lichen planus, pityriasis rubra pilaris or a drug reaction. Primary erythroderma is less frequent and is usually seen in cases of cutaneous T-cell lymphoma, in particular in Sézary's disease.

The most common causes of exfoliative dermatitis are best remembered by the mnemonic device ID-SCALP. The causes and their frequencies are as follows:

- Idiopathic - 30%

- Drug allergy - 28%

- Lymphoma and leukemia - 14%

- Atopic dermatitis - 10%

- Psoriasis - 8%

- Contact dermatitis - 3%

- Seborrheic dermatitis - 2%

Differential diagnosis in patients with erythroderma may be difficult.

Common causes of rashes include:

- Food allergy

- Medication side effects

- Anxiety

- Allergies, for example to food, dyes, medicines, insect stings, metals such as zinc or nickel; such rashes are often called hives.

- Skin contact with an irritant

- Fungal infection, such as ringworm

- Balsam of Peru

- Reaction to vaccination

- Skin diseases such as eczema or acne

- Exposure to sun (sunburn) or heat

- Friction due to chafing of the skin

- Irritation such as caused by abrasives impregnated in clothing rubbing the skin. The cloth itself may be abrasive enough for some people

- Secondary syphilis

- Poor personal hygiene

Uncommon causes:

- Autoimmune disorders such as psoriasis

- Lead poisoning

- Pregnancy

- Repeated scratching on a particular spot

- Lyme Disease

- Scarlet fever

The causes of nail psoriasis are unknown. It has been suggested that fungi may play a role.

Following effective treatment recurrence is common (10–50%).

Nail fungus can be painful and cause permanent damage to nails. It may lead to other serious infections if the immune system is suppressed due to medication, diabetes or other conditions. The risk is most serious for people with diabetes and with immune systems weakened by leukemia or AIDS, or medication after organ transplant. Diabetics have vascular and nerve impairment, and are at risk of cellulitis, a potentially serious bacterial infection; any relatively minor injury to feet, including a nail fungal infection, can lead to more serious complications. Infection of the bone is another rare complication.

The Nail Psoriasis Severity Index (NAPSI) is a numeric, reproducible, objective, simple tool for evaluation of nail psoriasis. It evaluates several signs separately, each on a 1–3 scale: pitting, Beau's lines, subungual hyperkeratosis and onycholysis. A 2005 study proposed a modified NAPSI scale for persons with psoriasis and named the title of their publication "Modification of the Nail Psoriasis Severity Index". Then, in 2007, a study found that there was a high level of inter-rater variability of the 2003 NAPSI scale and proposed another index which was, like the 2005 article, a modification of the 2003 article, and was named modified NAPSI.

A 2008 study found that Cannavo's qualitative system correlated with NAPSI (P<0.001) and is less time-consuming.

There is a risk of misdiagnosis with onychomycosis.

Subacute cutaneous lupus erythematosus (SCLE) is a clinically distinct subset of cases of lupus erythematosus that is most often present in white women aged 15 to 40, consisting of skin lesions that are scaly and evolve as polycyclic annular lesions or plaques similar to those of plaque psoriasis.

Characteristically the lesions appear in sun-exposed areas such as the vee of the neckline or the forearms, but not the face. It may be brought on by sun-sensitizing medications, but is usually associated with autoimmune disorders such as rheumatoid arthritis and Sjögren's syndrome.

Treatment generally involves sun avoidance and protection and topical corticosteroids. Sometimes systemic drug treatment is necessary. Besides corticosteroids other immunosuppressants such as methotrexate are also used.

Lesions of SCLE may have an annular configuration, with raised red borders and central clearing.

Psoriatic erythroderma (also known as erythrodermic psoriasis) represents a generalized form of psoriasis that affects all body sites, including the face, hands, feet, nails, trunk, and extremities. First-line treatments for psoriatic erythroderma include immunosuppressive medications such as methotrexate, acitretin, or ciclosporin.

Tetter refers to any skin condition characterized by reddish vesicular eruptions and intense itching. Common diseases called tetter include:

- Eczema and Duhring's disease

- Herpes

- Porphyria cutanea tarda (PCT)

- Psoriasis

- Ringworm and jock itch

A papulosquamous disorder is a condition which presents with both papules and scales, or both scaly papules and plaques.

Examples include psoriasis, lichen planus, and pityriasis rosea.

Dandruff can have several causes, including dry skin, seborrhoeic dermatitis, not cleaning/scrubbing often enough, shampooing too often, psoriasis, eczema, sensitivity to hair care products, or a yeast-like fungus. Dry skin is the most common cause of flaking dandruff.

As the skin layers continually replaces itself, cells are pushed outward where they die and flake off. For most individuals, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially in the scalp. It is hypothesized that for people with dandruff, skin cells may mature and be shed in 2–7 days, as opposed to around a month in people without dandruff. The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish flakes on the scalp, skin and clothes.

According to one study, dandruff has been shown to be possibly the result of three factors:

1. Skin oil commonly referred to as sebum or sebaceous secretions

2. The metabolic by-products of skin micro-organisms (most specifically Malassezia yeasts)

3. Individual susceptibility and allergy sensitivity.

According to a 2016 study, bacteria (mainly Propionibacterium and Staphylococcus) are more important to dandruff formation than fungi. Bacterial presence was in turn influenced by water and sebum amount.

Older literature cites the fungus "Malassezia furfur" (previously known as "Pityrosporum ovale") as the cause of dandruff. While this species does occur naturally on the skin surface of both healthy people and those with dandruff, in 2007 it was discovered that the responsible agent is a scalp specific fungus, "Malassezia globosa", that metabolizes triglycerides present in sebum by the expression of lipase, resulting in a lipid byproduct oleic acid. During dandruff, the levels of "Malassezia" increase by 1.5 to 2 times its normal level. Oleic acid penetrates the top layer of the epidermis, the stratum corneum, and evokes an inflammatory response in susceptible people which disturbs homeostasis and results in erratic cleavage of stratum corneum cells.

Neutrophilic dermatosis of the dorsal hands (also known as "Pustular vasculitis of the dorsal hands") is a skin condition that presents with edematous pustular or ulcerative nodules or plaques localized to the dorsal hands.