Iodine

What it actually measures

Iodine has essentially one job in your body, and it is a job nothing else can do.

Thyroid hormone is built by attaching iodine atoms to a protein scaffold. T4 carries four iodine atoms, T3 carries three, and the numbers in their names are literally the iodine count.

So without iodine you cannot make thyroid hormone at all, and thyroid hormone sets your metabolic pace, your body temperature and a great deal else besides.

This is why iodine deficiency was historically one of the most consequential nutritional problems in the world, causing goitre, hypothyroidism and, in pregnancy, preventable cognitive impairment in children.

Salt iodisation is one of the genuine public health successes of the last century, and it is why frank deficiency is now uncommon in countries that adopted it.

Where things get complicated is not the biology. It is the measurement.

Why a single test tells you almost nothing

Most iodine leaves your body in urine, so urinary iodine is the logical marker. And at population level it works well.

A 2012 review in Nutrition Reviews described median urinary iodine concentration in spot samples as the objective biomarker of population iodine status, with a cut-off of 100 micrograms per litre.

The same review flagged the problem with extending that to individuals: applying the crude proportion of values below the cut-off to individuals has led to overestimating the prevalence of iodine deficiency.

The direct evidence for why is stark. A 2011 study in The Journal of Nutrition collected 341 twenty-four hour samples and 177 spot samples from 22 women over 15 months.

It found intra-individual coefficients of variation of 32 percent for measured 24-hour excretion, 33 percent for estimated 24-hour excretion, and 38 percent for spot concentration.

Its conclusion is the title of the paper: ten repeat collections, spot or 24-hour, are needed to reliably estimate one person's iodine status to 20 percent precision.

Ten collections. That is what it takes for the test most people assume answers the question in one sample.

So the honest position is that iodine status is genuinely hard to measure in an individual, and a single urinary iodine result should carry far less weight than it usually does.

The 2012 review noted dried-blood-spot thyroglobulin as an additional sensitive functional biomarker, which is a research and population tool rather than something you can readily order.

What moves it

Down: low intake, particularly in regions without salt iodisation. Using non-iodised salt, which includes most sea salt, pink salt and the salt in processed food. Diets excluding dairy, fish, eggs and seaweed. Pregnancy and breastfeeding, which raise requirement substantially. And goitrogens in very large quantities, which matters far less than the internet suggests at normal intakes.

Up: iodised salt, dairy, fish, eggs and shellfish. Seaweed, sometimes dramatically, since kelp in particular can contain extremely high and variable amounts. Iodine supplements, including high-dose products sold for thyroid support. Some medications, notably amiodarone. And iodinated contrast used in imaging, which can raise it substantially for weeks.

The honest hedge

I am not a doctor and I am not diagnosing anyone, and this is a nutrient where the enthusiasm outruns the evidence in a specific and risky way.

High-dose iodine supplementation is promoted for thyroid support, and excess iodine can cause thyroid dysfunction in both directions. It can trigger hypothyroidism in susceptible people, and hyperthyroidism in others, particularly those with nodules.

Very high intake is also associated with thyroid autoimmunity, which matters given that TPO antibodies are how autoimmune thyroid disease is identified.

So megadosing iodine to support a thyroid is a genuinely poor idea, and I would say that more firmly than I say most things on this site.

Kelp and seaweed supplements deserve a specific mention because the iodine content varies enormously between products and batches, which means the dose is unknown even when someone thinks they are being careful.

In pregnancy, iodine requirement rises substantially and adequacy genuinely matters for the developing brain. That is an obstetric provider conversation, not a self-directed supplementation decision.

And given the measurement problem, this is a nutrient where getting intake roughly right through food beats trying to test your way to precision.

So. What to actually do.

Tonight, at no cost. Check what salt you use. Sea salt and pink salt are generally not iodised, and if you switched away from ordinary table salt you may have quietly removed your main source.

Count your food sources. Dairy, fish, eggs and shellfish are the reliable ones alongside iodised salt.

Do not chase a single urinary iodine result. Ten repeat collections are needed for 20 percent precision in one person, so a single value carries far less information than it appears to.

Be careful with kelp and seaweed supplements. Iodine content varies enormously between products and batches, which makes the actual dose unknown.

Do not megadose iodine for thyroid support. Excess causes dysfunction in both directions and is associated with thyroid autoimmunity.

If pregnant or planning to be, raise it with your obstetric provider. Requirement rises substantially and adequacy matters for the developing brain.

Test the thyroid rather than the mineral. A full thyroid panel with free T3, free T4 and antibodies tells you what your thyroid is actually doing.

You are not going to test your way to certainty on this one. You are going to get the intake roughly right from food, and let the thyroid panel tell you whether it worked.

Your body is not broken. It is blocked. And with iodine the honest answer is that the block is rarely the mineral and the measurement is rarely the answer.

Go check what salt is in your cupboard.