Potassium iodide is an inorganic compound with the chemical formula KI.
Potassium iodide is used to loosen and break up mucus in the airways. This helps you cough up the mucus so you can breathe more easily if you have long-term lungproblems (e.g., asthma, chronic bronchitis, emphysema). This medication is known as an expectorant.
Potassium iodide is also used along with antithyroid medicines to prepare the thyroid gland for surgical removal, to treat certain overactive thyroid conditions (hyperthyroidism), and to protect the thyroid in a radiation exposure emergency. It works by shrinking the size of the thyroid gland and decreasing the amount of thyroid hormones produced.
Contributes to overall healthy thyroid function:
Potassium iodide is known as the thyroid mineral. It helps in keeping iodine levels in the thyroid in proper balance. When iodine levels are not optimal, it can lead to both hyper or hypothyroidism, two conditions whereby the thyroid gland produces too much or too little iodine. For this reason, potassium iodide acts as a powerful treatment and preventative mechanism for both forms of thyroid disorder through a mechanism of counteracting any less-than-optimal iodine levels due to dietary deficiencies. Potassium iodide also boosts colloid accumulation in the thyroid follicles, contributing to more healthy functioning in the important gland.
This mineral is also used prior to thyroid surgery as a preparatory mineral for the gland by reducing thyroid vascularity.
Iodine deficiency may be related to arthritis, and potassium iodide may help prevent this. 
Iodine deficiency has also been related to emotional disorders and malfunctioning glandular systems, and weight gain associated with hormonal imbalance. Dr. David Brownstein also believes that iodine deficiency may be linked to breast, ovarian, uterus and prostate cancers and cysts.
Nausea, vomiting, stomach ache, diarrhea, metallic taste in the mouth, fever,headache, or acne may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.
If your doctor has directed you to use this medication, remember that he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.
Tell your doctor right away if any of these unlikely but serious side effects occur: burning mouth/throat, sore teeth/gums, swelling inside the mouth, increased saliva,eye irritation/swollen eyelids, severe headache, swelling of the front of the neck/throat (goiter), signs of decreased thyroid gland function (e.g., weight gain, cold intolerance, slow/irregular heartbeat, constipation, unusual tiredness), confusion, numbness/tingling/pain/weakness of the hands/feet.
Seek immediate medical attention if any of these rare but very serious side effects occur: chest pain, black stools, vomit that looks like coffee grounds, bloody diarrhea.
A very serious allergic reaction to this drug is rare. However, seek immediate medical attention if you notice any symptoms of a serious allergic reaction, including: rash,itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing, fever with joint pain.
None are recorded.
Source: Wikipedia, https://en.wikipedia.org/wiki/Potassium_iodide