Potassium iodide (KI) is a medication and dietary supplement. As a dietary supplement, it has utility in patients with low iodine intake, a more frequent occurrence in developing countries. As a prescribed medication, it is used to treat severe hyperthyroidism, cutaneous inflammatory dermatoses, nuclear emergencies, and to protect the thyroid gland when using radiopharmaceuticals.

In severe hyperthyroidism or refractory hyperthyroidism, patients receive KI for short-term use in the following situations[1][2][3][4][5]:

• In the preoperative preparation for thyroidectomy in Graves disease.
• Thyroid storm, because iodine blocks the release of T4 and T3 from the gland within hours.
• As serves as adjunctive therapy for Graves disease, used in combined treatment with antithyroid drugs and KI improves the short-term control of Graves hyperthyroidism.  Furthermore, it is helpful after the administration of radioiodine in Graves disease, especially in patients who wish to avoid taking or who are allergic to thionamides.

Concerning emergency radiation, the U.S. Nuclear Regulatory Commission (NRC) and the American Thyroid Association (ATA) require states to consider including KI as a protective measure. Its utilization is necessary when within a 10-mile radius of a nuclear along with adequate prevention methods such as evacuation, sheltering, and avoiding contaminated foods in the event of a nuclear accident. Furthermore, they state that KI must be available to state and local governments.[6] The guidance titled "Potassium Iodide as a Thyroid Blocking Agent in Radiation Emergencies" from the Food Drug Administration (FDA) of the United States prioritizes age, which is the primary factor for determining risk for radioiodine-induced thyroid cancer after radiation exposure. Those at highest risk are infants, children, and pregnant and nursing females.  The recommendation is to treat this population at the lowest threshold of the predicted radioactive dose to the thyroid.  Any person over 18 years old and up to 40 years old should receive treatment at a slightly higher limit.  Lastly, anyone over 40 years old should have KI treatment only if the predicted exposure level is high enough to destroy the thyroid, inducing lifelong hypothyroidism. KI works best if used within 3 to 4 hours of exposure. In the event of a nuclear accident, KI pills, taken once daily, decrease thyroid uptake of radioactive iodine. It almost protects the thyroid completely if administered within 12 hours before radioactive iodine exposure; after exposure, the degree of protection declines (80, 40, and 7 percent after 2, 8, and 24 hours, respectively).[7]

In regards to patients with dermatoses, the two best indications in this group are neutrophilic dermatoses and panniculitis. Especially for lymphocutaneous and cutaneous sporotrichosis, itraconazole is the drug of choice for the treatment. However, patients who don't respond to itraconazole at 200 mg/day, can receive KI together with other antimycotics as an alternative.[8] It is also successfully used for other inflammatory dermatoses. For instance, erythema nodosum, subacute nodular migratory panniculitis, nodular vasculitis, erythema multiforme, and Sweet syndrome.[9]

KI has several mechanisms of action on thyroid function. In euthyroid patients, the iodine has two effects at two different times. The most rapid (hours to days) effect, at pharmacologic doses of KI, is to decrease thyroglobulin proteolysis, thereby decreasing thyroid hormone secretion. The resulting slight reductions of T4 and T3 concentrations in serum cause transient increases of thyrotropin (TSH) concentrations in serum.[10] Secondly, KI inhibits thyroid hormone synthesis. The administration of KI leads to temporary inhibition of iodine organification in the thyroid gland, thereby decreasing thyroid hormone biosynthesis, a phenomenon called the Wolff-Chaikoff effect (WCE). However, within two to four weeks of continual exposure to excess iodine, organification, and thyroid hormone biosynthesis resume in a normal fashion, which is called escape from the Wolff-Chaikoff effect.[11][12] This phenomenon is produced by lower iodide uptake during the escape from the acute Wolff–Chaikoff effect. It results from a decrease in Na+/I– symporter (NIS) expression.[13] Exist abnormal autoregulation of the iodine in the autoimmune thyroid disease. The iodine organification persists and can result in or exacerbate hypothyroidism in patients with Hashimoto thyroiditis, or ameliorate hyperthyroidism in Graves disease. Thus, patients with Graves hyperthyroidism are more sensitive than normal subjects to the inhibitory effect of pharmacologic doses of iodine, making iodine treatment effective in some patients. Also, pharmacologic amounts of iodine may acutely ameliorate hyperthyroidism by blocking thyroid hormone release.[4] Furthermore, it is used in preparation for thyroidectomy because it decreases the vascularity of the thyroid gland. Therefore, this decreases the risk of post-thyroidectomy hemorrhaging.[1][2] KI should be administered at least one hour after the administration of thioamides to prevent new hormone synthesis since the new iodine substrate.

In the event of a nuclear accident, taken once daily, KI can decrease the mortality and morbidity of thyroid cancers provoked by radioactive iodine exposure; this is because it directly blocks the radioiodine uptake in the thyroid gland. KI floods the thyroid with non-radioactive iodine, preventing the uptake of the radioactive molecules, which subsequently get excreted in the urine.[14]

The precise mechanism by which KI acts against inflammatory dermatoses is unknown. The dermatoses treatable with KI usually display neutrophils in the early stages. Research demonstrates that iodine, as well as dapsone, can suppress the production of toxic oxygen intermediates by polymorphonuclear cells and thus exert its anti-inflammatory effect.[15] The precise mechanism by which KI kills fungi is also unknown. It is unclear whether KI works against fungi by a fungicidal mechanism or by enhancing the body's immunologic and nonimmunologic defense mechanisms. However, it is possible to assume that it has an important anti-inflammatory role, since the patients that show better response also present systemic symptoms and increased C-reactive protein. It usually improves fast, with fever, pain and erythema reduction in two days and complete remission in up to two weeks.[9]

The dose of KI used to treat dermatoses is much higher than that in thyrotoxicosis (250 mg 3 times daily) or in radiation (100 to 150 mg single dose). Physicians typically begin treatment of inflammatory dermatoses with an oral dosage of 300 mg (approximately six drops of supersaturated potassium iodide (SSKI)) 3 times daily, followed by weekly increases as tolerated. In the case of mycoses, the administration is often higher, beginning at 600 mg (approximately 12 drops of SSKI) orally three times each day and often increased to 6 g (about 127 drops of SSKI) daily if tolerated.

Most presentations are given orally, usually with juice or milk, to protect against gastrointestinal irritation. However, there are some exceptions. There are several FDA-approved KI products, including tablets (65 and 130 mg) and oral solutions (65 mg/mL).

Additionally, there exist another two liquid presentations prescribed orally:

• SSKI with 35 to 50 mg of iodine per drop and KI with about 24 mg per drop. It is usually administrated orally and mixed with juice or milk due to the bitter taste, especially in infants. 
• Potassium iodide-iodine (Lugol solution [5 to 8 mg of iodine per drop]) is usually given orally with the recommended dosage of 3 to 5 drops three times daily. Although iodine is typically well-tolerated, reports exist of local esophageal or duodenal mucosal injury and hemorrhage, particularly in the treatment of thyroid storm.[16][17] For patients unable to take oral medication, Lugol solution can be added directly to intravenous fluids because it is sterile.[18] An alternative is to give the iodine solution per rectum.[19]

Adverse effects are unlikely when KI is used at low doses and for a short time (less than two weeks). The most common side effects are on the digestive system, predominantly being gastrointestinal intolerance and its bitter (metallic) taste; thus, the recommendation is that it be taken with juice or milk to protect against gastrointestinal irritation.[9] However, there may occur significant side effects when high doses are administered, especially for the treatment of infectious skin disorders.

The acute side effects include diarrhea, nausea, vomiting, and stomach pain that can be ameliorated with gastrointestinal protection, and by avoiding rapid dosage increases. Nevertheless, prolonged use can cause Iodism or potassium toxicity. Iodism is an iodide poisoning syndrome characterized by soreness of the teeth and gums, severe headache, conjunctival hyperemia, lacrimation, blurred vision, rhinorrhea, and sialorrhea.  Concurrent use of KI with impaired renal function or other potassium-containing medications, potassium-sparing diuretics, and angiotensin-converting enzyme inhibitors (ACE inhibitors) may result in hyperkalemia.[8][9]

Because the patients receive large amounts of iodine in the drug, it could affect the metabolism of the thyroid gland. It can produce a WCE and produce hypothyroidism. However, there are autoregulation mechanisms that help maintain the normal function of the gland in euthyroid patients. The imbalance of thyroid hormones occurs when autoregulation is defective or absent. If it is just defective, the resulting WCE is inevitable, TSH increases, and hypothyroidism and goiter ensue. Failure to escape this condition, with resulting hypothyroidism, can result from with the administration of KI in patients with Hashimoto's thyroiditis, euthyroid patients previously treated by thyroid surgery or radioactive iodine for Graves' disease, patients taking certain drugs that inhibit thyroid function (e.g., lithium, phenazone, and, possibly, sulfonamides), patients previously treated with interferon alfa for chronic viral hepatitis,[20] and patients with a history of amiodarone-induced thyrotoxicosis, subacute thyroiditis, or Graves disease. When autoregulation is absent, Jod-Basedow disease occurs. The absence of autoregulation is typically only seen in areas where iodine deficiency with long-standing goiters occur. This alteration produces an excess of thyroid hormone resulting in thyrotoxicosis.[9]

Allergic reactions such as angioedema and urticaria should be a consideration during the administration of KI, like any drug. The use of KI can also cause an uncommon lesion in the skin called Ioderma, which is characterized by a severe acneiform, vesicular pustular, hemorrhagic, or urticarial lesions. Other systemic side effects of SSKI include urticaria, fever, eosinophilia, jaundice, pruritus, angioedema, and bronchospasm. In this case, the treatment is high-dose corticosteroid therapy.[21]

KI is contraindicated in patients who have thyroid disease or are using any drug that could alter the thyroid function.[22] Contraindications also include patients with an allergy to iodine. Clinicians should avoid giving it to patients with chronic renal failure because of the presence of potassium. Furthermore, it should be avoided in patients using potassium-sparing diuretics or angiotensin-converting-enzyme inhibitors to prevent hyperkalemia.[23] Immunocompromised patients such as patients with cancer, cirrhosis, AIDS, and autoimmune diseases, or poorly managed diabetics, transplant patients, and those using corticosteroids should not use KI because it affects the immune system.[23] It should not be indicated in pregnant or nursing women because it causes neonatal hypothyroidism, thyromegaly, fetal airway obstruction, and prolonged labor. Also, it is a pregnancy category D drug.[9]