There are several types of psoriasis that plaque-type psoriasis, occurring in 85%–90% of affected patients, is the most common form of psoriasis. Guttate psoriasis occurs in about 10% of patients, and erythrodermic and pustular types each occur in fewer than 3% of patients . In our study, chronic plaque-type psoriasis was seen in 98% of cases, and postular type in 2% of cases. Previous studies reported that the prevalence of a positive family history of psoriasis in first-degree relatives ranged from 4.5 to 91% . In the current study, a positive family history of psoriasis in first degree relatives was present in 79% of the cases.
There was a lack of agreement in the published studies about whether the prevalence of psoriasis differed between men and women. The results of a recent meta-analysis showed that there are no differences in the frequency of psoriasis between genders were found in the United States, United Kingdom, Norway, Spain, Scotland, and Taiwan in individuals of all ages combined. On the other hand, other studies reported a slightly higher prevalence of psoriasis in female subjects than male subjects in Swedish and Germany. In contrast, psoriasis was more frequent in men than in women in Denmark, Australia, Sweden, and China . In the present study, approximately three-fourths of patients with psoriasis had less than 45 years of age, and the majority of patients were seen in the age range of 25 to 35 years. The findings were in line with those reported in other studies. Recent reports indicate that the two-peak age of onset is considered for psoriasis; the early age of onset is between 20 to 29 or 30 to 39 years of age, and the second with a peak value around 60 years, after which the prevalence reduced. Psoriasis is uncommon before the age of 9 years .
The results observed in the present study showed a significant reduction in serum zinc level in psoriasis patients as compared to healthy subjects. We hypothesize that this could have happened for two reasons: first, it is possible that the decrease in serum zinc level in psoriasis patients may exacerbate the disease. Secondly, the progression of psoriasis may be associated with a reduction in serum zinc level. In agreement with the findings of our investigation, some researchers have reported significantly low serum zinc levels in psoriatic patients [19-21], although other authors found no such difference [22, 23]. For example, a study in Germany found that in contrast to the data given in the literature, there was no significant difference between serum zinc level in patients with atopic dermatitis and psoriasis Vulgaris compared with healthy subjects. They suggested that zinc replacement therapy in patients with atopic dermatitis and psoriasis appears to be indicated only in those with a documented zinc deficiency . In our study, a reverse relationship was observed between serum zinc level and the extent of a skin lesion in patients with psoriasis. Indeed, a greater extent of skin involvement was correlated with lower serum zinc levels. This is in agreement with the results of Nigam , who showed that patients with more than 20% body surface area involvement were significantly decreased serum zinc concentration compared with those with less than 20% body surface area involvement.
In order to determine the relationship between plasma zinc level and the extent of skin involvement in psoriasis, McMillan, and Rowe  evaluated plasma zinc, serum albumin, and alkaline phosphatase (a zinc-dependent enzyme) were measured in 35 psoriatic patients as compared with age- and sex-matched healthy controls. There was no significant difference between these two groups as a whole for plasma zinc level, but psoriatic patients with less than 10% body surface area involvement had significantly higher mean plasma zinc levels compared with the control group. In addition, a significant relationship was observed between the extent of surface involvement and the plasma zinc level, those with more than 10% body surface area involvement having lower levels than those with less than 10% body surface area involvement. These findings are consistent with the results of our study . Some researchers have demonstrated that the zinc content of neutrophils is significantly reduced in patients with psoriasis compared with both healthy subjects and patients with seborrhoeic dermatitis, which this reduction was unrelated to the extent of skin involvement . However, zinc levels of plasma and erythrocyte were unchanged.
Zinc supplementation can have a valuable role in patients with psoriasis. It has been suggested that oral zinc sulfate at a total dose of 120 mg/day of elemental zinc, for 6 months may be an effective and well-tolerated disease-modifying anti-rheumatic drug (DMARD) in psoriatic arthritis . In a previous study, the researchers found that oral zinc sulfate seems to be valuable in the treatment of psoriatic arthritis . In addition, another study evaluated the efficacy of a topical formulation of 0.25% zinc pyrithione in an emollient base compared with an emollient alone in the treatment of patients with localized psoriasis involving less than 10% of body skin areas. However, the results observed in this study showed a topical formulation of zinc pyrithione can be used to treat localized psoriasis . These findings are not consistent with the results of the present study. Our study showed that oral zinc sulfate (220 mg twice daily) is not effective in psoriasis patients with serum zinc deficiency. In line with this result, Voorhees et al. , reported oral zinc therapy in psoriasis no better than placebo though it did increase zinc concentration in psoriatic scales, uninvolved skin, and urine. In addition, Housman et al.  found topical 0.25% zinc pyrithione spray does not appear to enhance the efficacy of clobetasol propionate 0.05% foam after 2 weeks of therapy. These findings support our study results.
Some investigators have reported an association between thyroid dysfunction and a variety of dermatological conditions . There is currently no evidence to support an association between thyroid dysfunction and psoriasis. The present study demonstrated thyroid dysfunction in 8% of patients with psoriasis compared with 7% in control subjects, however; the difference was not statistically significant. In line with this result, Gul et al. , reported thyroid autoimmunity in patients with psoriasis who did not have arthritis was no different from that found in healthy individuals. Most researchers have reported that there is no connection between thyroid dysfunction and psoriasis. For example, a study by Vassilatou et al.  evaluated prospectively 114 psoriatic patients, 30 of them with psoriatic arthritis, and 286 subjects without psoriasis or known thyroid disease or autoimmune disease. Autoimmune thyroiditis in psoriatic patients was not related to the age of psoriasis onset, psoriasis duration, PASI score, psoriatic arthritis, and obesity. They eventually concluded that psoriatic patients with or without psoriatic arthritis do not have an increased risk for autoimmune thyroiditis. These findings are consistent with the results of our study.
Another study  evaluated prospectively 100 psoriatic patients compared with a control group of 54 patients, without known thyroid gland abnormalities. These authors found no significant difference in the thyroid gland functions between the psoriatic and the control patients. They observed that in patients with severe psoriasis, there are increased TSH levels and positive auto-antibodies titer compared to patients with mild psoriasis. In contrast, some researchers have reported thyroid function follow-up and suitable treatments should be performed regularly in female patients at high risk, especially those who have thyroid-stimulating hormone within the normal range but at the higher limit, those with positive anti-thyroid peroxidase antibodies, and those who have a small hypoechoic thyroid pattern in ultrasound .
The present study has several limitations that should be considered. First, this study was a single-center with a limited sample size and these results may not be generalizable to other centers. Large-scale population studies are necessary in order to confirm these observations. Second, Serum zinc level measurement may be affected by several factors, including stress, infection, or other metabolic conditions .