Iodine

The usual test is a spot URINE iodine, where the WHO bands of 100 to 199 ug/L adequate, 200 to 299 above requirements and 300 or more excessive are designed for population surveys and swing widely from day to day in one person; serum iodine is rarely measured and has a different range of roughly 40 to 90 ug/L; many labs also print iodine per gram or mmol of creatinine.

Iodine is the trace element the thyroid gland uses to build thyroid hormone. It comes from milk, dairy, fish, eggs and, in some countries, iodised salt. Because nearly all the iodine eaten is passed out in the urine within a day or two, a urine iodine test reflects the last few days' intake rather than the body's long-term status, and it is designed for surveying populations rather than judging one person.

It is requested mainly in pregnancy planning and pregnancy where intake may be low (vegan or dairy-free diets), in people with an enlarged thyroid or unusual thyroid results, before some thyroid scans and treatments, and after exposure to a lot of iodine from contrast dye, amiodarone or seaweed supplements. It is on some private nutrition panels.

• Contrast dye from CT scans and some X-rays, for weeks afterwards
• Amiodarone, some cough medicines, iodine-containing antiseptics and seaweed or kelp supplements
• A meal rich in fish, seaweed or dairy in the past day
• Iodised salt and fortified foods
• Some multivitamins and pregnancy supplements

• A diet low in dairy, fish and eggs, including many vegan and dairy-free diets
• Pregnancy and breastfeeding, where the need is higher
• A dilute urine sample after drinking a lot
• Low intake over the previous few days, for any reason

• A single spot urine swings widely with what was eaten and drunk that day, so one value says little about long-term intake
• A recent CT scan with contrast raises it for weeks
• Dilute or concentrated urine moves the concentration, which is why some labs correct it to creatinine

Usually a spot urine sample, sometimes corrected to creatinine; a 24-hour collection is more accurate but rarely done. Recent contrast dye, seaweed or iodine supplements should be noted. Serum iodine is rarely measured in the UK.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.