Iodine

Hair iodine reports the total amount of iodine in a sample of cut scalp hair, expressed as micrograms per gram of hair (µg/g). It is a research-stage nutritional marker. There are no validated clinical cutpoints or treatment targets, and using the result to guide supplements has not been shown to improve health.

Hair grows over weeks to months, but iodine can also collect on its surface or wash out. Those processes make it difficult to interpret the result as an average of your intake over that time.

Where Hair Iodine Comes From

Your body needs an outside supply of iodine. Food, water, and supplements provide it, and some medicines and medical contrast agents contain substantial amounts. Dietary iodine passes from your digestive tract into your blood. Your thyroid takes it from the blood, and most dietary iodine eventually leaves through urine.

Inside the thyroid, iodine is attached to thyroglobulin. This protein helps the gland make and store thyroid hormones. The thyroid hormones T4 and T3 regulate energy use and support brain development before birth and during childhood. The iodine measured in hair includes material incorporated during growth and material deposited from outside the hair shaft.

What Studies of Hair Have Found

A 2026 Japanese study measured scalp hair iodine in 778 adults. Higher hair concentrations had weak associations with estimated dietary iodine intake, blood iodine, and estimated iodine loss in urine over a day. That supports studying hair as a possible population marker, but the study did not establish how accurately it identifies deficiency or excess in an individual.

Hair iodine also varied with age and sex in that study, with higher concentrations in men. These findings do not justify a universal correction for sex or a personal nutrition target.

Goiter in Children

Goiter is an enlarged thyroid gland. In a study of 68 Moroccan children, those with goiter had lower hair iodine than children without it. A study of Kazakh schoolchildren found the opposite: hair iodine was about two thirds higher in the goiter group and was associated with larger thyroid volume.

Both studies were observational. They did not establish that the hair differences caused the enlargement, or that measuring several hair minerals together could identify its cause. Iodine deficiency, excess exposure, and other thyroid diseases can cause goiter; these studies do not provide a reliable way to distinguish them using hair.

Thyroid Function at Both Extremes

The evidence that too little or too much iodine can disturb thyroid function comes mainly from studies of dietary intake, urine, and thyroid blood tests. Those are different measurements from hair iodine, so their risk estimates cannot be assigned to a hair result.

TSH is the hormone signal that tells your thyroid to make hormones. Free T4 is the portion of T4 in blood that is not attached to proteins.

Subclinical hypothyroidism means TSH is elevated while free T4 remains normal. Symptoms may be present or absent.

A pooled analysis of three studies that compared populations at one point in time found about 2.8 times the odds of an underactive thyroid with reduced hormone levels in adults with excess iodine exposure compared with adequate exposure. The odds of subclinical hypothyroidism were roughly doubled. These estimates mainly concerned excess iodine from water or iodized salt and were based on limited, low-quality observational evidence.

In another study of 13,487 Chinese adults, higher urinary iodine measures were associated with more subclinical hypothyroidism, while lower measures were associated with more thyroid nodules. The study measured existing conditions at one point in time, so it could not establish whether changing intake would prevent them.

Large iodine exposures from supplements, the heart-rhythm medicine amiodarone, or CT contrast dye can cause an underactive or overactive thyroid in susceptible people. Existing thyroid disease increases susceptibility. An underactive thyroid can also cause widespread hair thinning, as described in a case report of an underactive thyroid caused by iodide exposure. Hair loss does not establish what an iodine concentration in hair means.

Long-Term Health Research

Studies linking urinary iodine to death rates have given different results. In a Spanish cohort followed for an average of 7.3 years, the group with the lowest urinary iodine had about 70% higher mortality than the reference group. In a US cohort followed for a median of 19.2 years, the highest urinary iodine category had about 19% higher mortality.

A Korean study of 5,497 adults found no statistically clear association between estimated iodine intake and death from any cause, cardiovascular disease, or cancer after adjustment for other factors. Its estimates were imprecise. These observational studies used urine measurements, and they neither establish causation nor show that hair iodine predicts longevity.

Autism and Neurodevelopment Research

Some observational studies have found lower hair iodine in autistic children. In an Arizona study, hair iodine was 45% lower in 51 autistic children than in 40 comparison children. A later meta-analysis also found lower hair iodine across the studies it pooled.

These group differences do not establish iodine deficiency, show that low iodine causes autism, or demonstrate that supplementation improves autism symptoms. A systematic review of candidate biomarkers for neurodevelopmental conditions found no marker whose strong diagnostic accuracy was replicated in independent studies. Hair iodine has no validated role in diagnosing autism or choosing its treatment.

When Results Can Be Misleading

• Washing: In an experiment involving people treated with radioactive iodine, detergent removed about half the tracer from hair roots. This shows that iodine can be loosely bound; it does not establish how much an ordinary shampoo removes from a routine sample.
• Surface contamination: Iodine-containing antiseptics or hair products can add iodine to the sample. Sweat and scalp oils can also contribute material on the hair surface.
• Hair treatments and collection: Dyeing, bleaching, chemical treatments, the sampling site, and the length collected can affect hair mineral analysis. A change in these conditions can complicate comparison with an earlier sample.
• Laboratory differences: Sample preparation and measurement procedures can affect results. A study that sent portions of one person's hair to commercial laboratories found conflicting mineral results and nutritional advice. That was a study of hair mineral panels generally, not an iodine-specific estimate of error.

What a Trend Can Mean

A change in hair iodine can reflect changes in intake, surface exposure, hair care, or laboratory variation. Studies have not established how large a change is clinically meaningful or whether a rising or falling result represents better thyroid health. Repeated measurements remain exploratory.