temperature in the luteal phase should really be up around 36.6 and higher.
Emma: These are fantastic white flags for low iodine, I also want to delve into testing because it is such a minefield.
Miranda: Oh yes, it ' s a minefield!
Emma: We have had many years of clinical practice. We have seen the patch test, and the spot urine versus the 24- hour debate over the iodine loading test. But can you talk us through what tests you use and any caveats for ensuring the most accurate results? And then can you explain what corrected iodine is?
Miranda: The iodine spot versus a 24- hour- let ' s look at that first. The 24-hour is preferable but practically it ' s quite tricky to be checking every urine sample over a 24-hour period. Urine is reflective of iodine intake over the previous 48 hours, and the understanding is that approximately 90 % of iodine is excreted in the urine.
Then, I did look at the iodine loading test where the patient has that massive dose of iodine and then has urine collected for the next 24 hours, and the patient is considered iodine deficient if less than 90 % of the administered dose is excreted. That is the whole iodine loading, or the whole challenge test.
But urinary iodine levels can take weeks or months to reflect changes. And some of the research I looked at showed that it ' s usually at that 30-hour point that we get the biggest dump of iodine excretion in the urine. So, you ' ve missed it if you ' re only testing up to 24 hours.
So, then I think the most important thing when we ' re getting a urinary iodine done is that it is corrected. And that means that the iodine that ' s measured in micrograms is divided by creatinine in millimoles, and that ' s to account for any dehydration, and then multiplied by 8.85. That is the corrected iodine in micrograms that we actually need. A lot of labs don ' t actually do that.
They generally do the iodine divided by creatinine, but it ' s whether they then multiply it by 8.85 that counts. That’ s what gives the most accurate testing, although no tests are actually 100 % accurate.
So, it ' s incumbent on us as practitioners to, first, make sure that corrected iodine is being measured and, secondly, ring the lab to double check. I’ ve found pathologists very happy to respond to this.
Emma: And with that urine testing for iodine, are there any caveats around when they should do it? What do you tell your patients?
Miranda: Okay, can I return to that one in just a second? Because one other thing I just wanted to say about creatinine is that we want to be measuring the patient ' s own creatinine levels, because some of the labs use a median creatinine instead of the patient ' s own actual creatinine value. So, again, it ' s making sure we have the iodine divided by the patient ' s own creatinine level and then multiplying that by 8.85. To return to your question, we would do a fasting morning, in the first morning void. And the question is, do we need to avoid particular high-iodine foods? I don ' t do this, simply because I want to know what ' s going on in patients’ normal routines.
Hydration status is really important though, and then, as we talked about, that measurement for creatinine to account for any hydration issues as well. Sometimes the readings can come out a little bit odd. So, hydration is really important when we ' re looking at these measurements. It’ s a test that can give us so much important information about the thyroid and some of the other things we ' ll get into as well.
Emma: Before we dive into dosage and forms, I want to just quickly touch on the RDIs, because the recommended daily iodine intake is 150 micrograms in adults who are not pregnant or lactating, while all pregnant women should consume approximately 250 micrograms of iodine per day. What do you think of those as a guide?
Miranda: What I find really interesting is the 250 micrograms daily recommended in pregnancy. We know that pregnancy is your thyroid ' s greatest stress test. We also know that, in pregnancy, thyroid antibodies can do all sorts of amazing things. Or, if a pregnant woman has not had her thyroid antibodies checked pre-pregnancy and does have pre-existing and existing thyroid antibodies during pregnancy, how do we feel about a dose of 250 micrograms of iodine or up to 270 micrograms of iodine in our prenatals? Because as we know, iodine makes thyroid antibodies more aggressive.
And yet in pregnancy, the foetus, as we talked about earlier, particularly in the first and third trimesters, needs that iodine supply for brain development, particularly in the first trimester, so where do we sit with that? There ' s a lot of iodine in seafood, and so we talk a lot about salmon for our patients. Seeing that we can get such a high exposure of iodine from diet alone it ' s making me really question supplementation levels now.
So, 150 micrograms of iodine is required for adequate thyroid hormone synthesis. That ' s a day. The median levels range between 100 to 199. And then in some of the research we were looking at whether 200 to 300 micrograms presents a risk of iodine-induced hyperthyroidism in susceptible groups. And then, would over 300 increase that risk? So, it’ s really interesting, the pregnancy thing.
Emma: Yes, it is, because we all just apply a generic recommendation and provide our pre-conception and pregnancy patients with standard pre-conception and pregnancy supplements with dosages of around the 270 micrograms. And what might be the cumulative effect here of supplementation plus dietary source if a woman has thyroid and positive
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