A useful application is in the preparation of aryl iodides from arenediazonium salts. KI, acting as a source of iodide, may also act as a nucleophilic catalyst for the alkylation of alkyl chlorides, bromides, or mesylates. History
Potassium iodide has been used medically since at least 1820. Some of the earliest uses included for syphilis. Chernobyl
Potassium iodides (KI) value as a radiation protective (thyroid blocking) agent was demonstrated following the Chernobyl nuclear reactor disaster in April, 1986. A saturated solution of potassium iodide (SSKI) was administered to 10.5 million children and 7 million adults in Poland as a preventative measure against accumulation of radioactive 131I in the thyroid gland. Reports differ concerning whether people in the areas immediately surrounding Chernobyl itself were given the supplement. However the US Nuclear Regulatory Commission (NRC) reported, "thousands of measurements of I-131 (radioactive iodine) activity...suggest that the observed levels were lower than would have been expected had this prophylactic measure not been taken. The use of KI...was credited with permissible iodine content in 97% of the evacuees tested. "With the passage of time, people living in irradiated areas where KI was not available have developed thyroid cancer at epidemic levels, which is why the US Food and Drug Administration (FDA) reported "The data clearly demonstrate the risks of thyroid radiation... KI can be used [to] provide safe and effective protection against thyroid cancer caused by irradiation. "Chernobyl also demonstrated that the need to protect the thyroid from radiation was greater than expected.
Arachnoid cysts are cerebrospinal fluid covered by arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the arachnoid membrane, one of the three meningeal layers that cover the brain and the spinal cord. Primary arachnoid cysts are a congenital disorder whereas secondary arachnoid cysts are the result of head injury or trauma. Most cases of primary cysts begin during infancy; however, onset may be delayed until adolescence. Signs and symptoms
Patients with arachnoid cysts may never show symptoms, even in some cases where the cyst is large. Therefore, while the presence of symptoms may provoke further clinical investigation, symptoms independent of further data cannot—and should not—be interpreted as evidence of a cysts existence, size, location, or potential functional impact on the patient.Symptoms vary by the size and location of the cyst(s), though small cysts usually have no symptoms and are discovered only incidentally. On the other hand, a number of symptoms may result from large cysts:

Cranial deformation or macrocephaly (enlargement of the head), particularly in children
Cysts in the suprasellar region in children have presented as bobbing and nodding of the head called bobble-head doll syndrome. Cysts in the left middle cranial fossa have been associated with ADHD in a study on affected children. Headaches. A patient experiencing a headache does not necessarily have an arachnoid cyst.