Kelp is seaweed  that grow underwater. They are used for many purposes like for medicine, food etc.
It is commonly eaten in Asian countries, such as Japan and is considered a superfood because of all the nutrients it has.
It has minerals like Magnesium, iron, zinc, etc. and vitamin profile includes niacin and riboflavin. It also has arsenic, a heavy metals, which can cause allergic reactions.
Kelp is used as a supplement for iodine and is used by the people having low-thyroid levels . It is a popular fat-burner supplement for weight loss. 
Possible Allergies to Kelp
Allergies occur when your immune system is triggered by environmental factors and causes immune cells to fight against these factors.
Some people are more prone to developing allergies because it’s what is encoded in their genes.
1. Allergies to Kelp as Inappropriate Immune Response
Although allergic reactions rise up instantly, sometimes they are not immediate; they can happen during or after digestion of the particular food item, in this case, the Kelp.
Allergic reactions can occur because of the contents of the particular food item.
One of the major allergic reactions to kelp (or any other seaweed) is Anaphylaxis. This condition can cause low blood pressure and blocked airways.
Major symptoms of allergies include abdominal pain, diarrhea, difficulty in swallowing and breathing, vomiting, itching on skin, etc.
2. Abnormal Thyroid Function after Consuming Kelp
One of the most important minerals that Kelp has is Iodine. Iodine plays a very important role in thyroid hormone formation and metabolism.
People dealing with Hypothyroidism (deficiency of thyroid hormone which leads to a decrease in metabolism) should consume kelp as it is abundant in Iodine.
However, too much Iodine can also cause many problems.
RDA for Iodine is 150 microgram per day, Kelp has more than this.
Taking kelp in the diet would no doubt help maintain the iodine levels in the body, but too much of it can lead to hyperthyroidism , plus your salt already helps much in that case.
Hyperthyroidism is a condition where the thyroid gland overproduces the hormone, thyroxin . Symptoms of hyperthyroidism include high metabolism, weight loss, etc.
3. Arsenic Toxicity due to Kelp Consumption
Kelp can absorb arsenic from water in which it grows. Arsenic is a heavy metal can lead to ‘Arsenicosis’ .
The acute symptoms include vomiting, watery diarrhea, and abdominal pain. Arsenic poisoning majorly happens through taking water from the water bodies highly polluted with it.
Arsenic goes into the water through soil, and the plants growing that in that polluted water (even the fishes) take that up in their bodies.
Arsenic is passed on into the body once kelp  that has grown in that polluted water, is ingested. Arsenic levels in the body can be detected through urine analysis. 
4. Laxative Effect of Kelp
Many people would be unaware of the fact that Kelp is a natural laxative  as it has ‘algin’ which is a high molecular weight polysaccharide.
Laxatives are good for the people having bowel movement problems, however, taking too much of it comes with a cost.
Excessive intake of laxatives can lead to diarrhea and loss of vital minerals from the body.
5. High Blood Pressure after Consuming Kelp
Excess of sea Kelp intake can increase the iodine levels in the body. In such a case, the thyroid gland traps more iodine from the blood and produces an excess of thyroid hormones.
Thyroid hormones increase Basal Metabolic Rate (BMR) along with an increase in blood pressure.
6. Hepatotoxicity after Kelp Intake
Although this situation is rare, sea kelp has reportedly caused Hepatotoxicity in some people .
In a case study, it was found out that woman suffering from Jaundice had been taking herbal tea containing Kelp for along time.
The patient was taking it as a treatment for Diabetes. The herbal tea intake was stopped during jaundice treatment and soon the bilirubin levels got back to normal.
7. Thrombocytopenia due to Kelp Intake 
Thrombocytopenia is a condition of low platelet count, cells in your blood that stick together to help it clot.
As a result of which, even during a small cut, bleeding can’t the controlled because the clotting system is damaged.
Several cases of autoimmune thrombocytopenia cases have been reported in that past as a result of kelp intake.
Kelp can easily concentrate heavy metals like arsenic and kelp preparations contain a substantial amount of this metal.
Arsenic toxicity can lead to bone marrow depression (immune system weakens) and change in the shape of red blood cells.
Although many cases of thrombocytopenia are congenital, an excess of kelp can worsen the situation.
Kelp is a staple in Asian cuisine especially Japan. It is available in various forms: powder, supplements, or even raw.
The easiest way to include kelp in the diet is to sprinkle kelp powder in your soups, smoothies or salads.
Raw kelp noodles can also be included in salads, wraps, or other side dishes.
Seaweed flakes can be used as a seasoning in a variety of dishes. Strips of kelp can be incorporated in soups and stews. Seaweed sheets can be used to prepare sushi.
It is best to include kelp in the diet. Kelp supplements should be taken only after consulting a health practitioner.
The daily requirement of iodine for adults is 150mcg.  Identify the iodine content of the kelp supplement you opt for.
FDA recommends that daily consumption of kelp should not provide more than 225 micrograms of iodine. 
Ensure that you get good quality kelp supplement which is free from heavy metal toxicity. Consult a health practitioner about the dosage of kelp supplements.
Kelp in the diet is great, but one should start with small quantities. 1 tablespoon of kelp powder offers 500 % of DRV of iodine.
Iodine though great for the brain and general health should be taken only as per daily requirement values. Excess iodine intake can have serious adverse effects.
Kelp supplements should be taken only after consulting the doctor.
Also, follow the manufacturer’s dosage instructions and check the amount of iodine present in the kelp supplement.
A study published in BMJ Case Reports, 2014 reported that 45-year-old women with no previous history of thyroid disease developed hyperthyroidism due to excessive iodine intake from kelp. 
Another case report pointed out the incident of iodine-induced hyperthyroidism in a 39-year-old woman who consumed kelp containing tea. 
High dose kelp tablets have also been found to cause hyperthyroidism. 
Ingestion of kelp supplement has been reported to affect blood health and cause dyserythropoietic and autoimmune thrombocytopenia. 
A recent study published in Korean Circulation Journal 2016, reported a case where middle-aged women experienced life-threatening cardiac side effects due to the consumption of kelp solution. 
The women aimed to treat hypertension with a kelp solution.
Excess iodine intake from kelp solution led to thyrotoxicosis which in turn affected heart health.
Kelp tea has also been found to have adverse effects on liver health. 
If the waters in which kelp is grown is contaminated, it is highly likely that kelp supplements made from such a source would have heavy metal contamination.
A case report revealed the potential of kelp supplements (containing heavy metal contamination) to cause arsenic toxicosis in a healthy individual. 
The number of case reports suggestive of kelp’s adverse effect is pretty high in comparison to some superfoods. This may make you wonder whether you should opt for kelp or no.
Here are a few precautionary measures to be considered while consuming kelp:
- If including kelp in diet, ensure you consume only small quantities and the kelp, you use, is a good source. Also, it is better to consume kelp occasionally rather than daily.
- Avoid kelp in a thyroid condition unless advised by a doctor.
- If consuming kelp supplement, find out the iodine content of a single capsule. Ensure you do not take more than 150-225mcg iodine per day.
- It is better to include kelp in the diet rather than taking a supplement.
- Kelp, whether raw or supplemental form, should be free from heavy metal contamination. Ensure you get your kelp from a reliable brand.
Sea kelp is a versatile underwater plant that is not only used as a food item but also in cosmetics like shampoos because of its protein content.
Kelp has been used in Iodine supplements and has been proven a boon for hypothyroidism patients but excess intake can lead to it can lead to problems.
Before its use in anything, it should be made sure that the surrounding or the source in which it grows is free of any pollutants or toxic metals.