Recently in our group & individual mentoring sessions we’ve been looking at lots of patients’ urinary iodine results.   Many of you will know that I’m a bit of a fan of doing spot urinary iodine testing to gain some understanding about patients’ iodine, in spite of several well-documented limitations of the test.  The first thing to remember is that urinary iodine has a diurnal rhythm, parallel to the rhythm seen with the thyroid hormones, so urinary values will fluctuate throughout the day.  We can get around this by always asking patients to collect the sample at the same time – preferably a fasting early morning urination, which represents the lowest iodine concentration in a day.  That way we know we’re always comparing apples with apples.  The second limitation and frequent cause for misinterpretation of results is not allowing for the concentration/dilution factor of the urine sample.  Because urinary iodine is measured as mcg/L of urine, if an individual has very concentrated urine (i.e. not much fluid for the amount of solutes), then the iodine value will appear artificially ‘greater’ than it really is.  In contrast, if my patient is well or even over-hydrated making their urine markedly dilute – suddenly the iodine content will appear very ‘poor’.  So how do we get around this?  Whenever we request a urinary iodine test we should also ask for urinary creatinine to be measured as well, many labs do this automatically, but you may need to ask other labs specifically for it – it shouldn’t cost any extra.  Urinary creatinine is in part a marker of urine concentration – high levels e.g. >14mmol/L, suggest highly concentrated samples and low e.g. <4mmol/L, very dilute.  Therefore to make urinary iodine interpretation much more accurate there is a mathematical formula you can use that considers the iodine and creatinine together – it is sometimes referred to as ‘Corrected Iodine’:

Iodine (mcg) ÷ Creatinine (mmol) X 8.85 = Corrected Iodine

e.g. patient’s random urinary iodine value was 52 mcg/L & creatinine was 6.2 mmol/L

–> (52 ÷ 6.2) X 8.85 = 74.2 mcg/gCR

Ideally we want most individuals to have a minimum urinary iodine of 100 mcg/gCR  and 150 mcg/gCR in pregnancy & lactation.  There is something else called an iodine:creatinine ratio which is quite different and is considered not as accurate – so take note not to mix up the terms!  I find random urinary iodine testing to be worthwhile in many of my patients – take one of my current patients who is currently using Lithium, a known goitrogen.  By checking her urinary iodine both at baseline and after initiation of iodine supplementation we can see whether the dose is really enough for her given the presence of this medication.  Then there’s fertility cases, and thyroid cases and breast pain and…and …and…. Yes, getting iodine status right is another fixation of mine… and there’s no dilution there! 🙂