Your thyroid can't make its hormones without iodine, and almost nobody tests for it.

Awareness of iodine is low, and it rarely appears on a standard blood test. Yet it underpins the thyroid hormones that regulate your cycle and drive your baby's brain development from early pregnancy. Unfortunately, we're starting to see goitre and underactive thyroids more often.

Why women are at risk

Requirements rise sharply in pregnancy and lactation to support fetal neurodevelopment, which is why iodine should be optimised before conception, not reactively. Some women are at even higher risk:

  • Those who are pregnant or breastfeeding

  • Those who eat little bread, dairy or seafood, including people who are gluten free, dairy free or vegan

  • Those who use non-iodised salt, such as sea salt or rock salt

Fortification has helped enormously. Since 2009, bread in both Australia and New Zealand has been made with iodised salt, and Australian data suggest the share of women with adequate intake rose from around 40% to 91%. But those fortification goals sit below what pregnancy and lactation need, and people who rarely eat bread may be part of the remaining 9%. Iodine status also tends to decline with age.

Too little, just right or too much? Making sense of your results

Iodine follows a U-shaped curve: too little and too much both carry risk, including thyroid dysfunction. The common test is urinary iodine concentration (UIC), which is best read as a population marker:

  • Too little: a median UIC below 100 µg/L when not pregnant, or below 150 µg/L in pregnancy. This raises the risk of goitre and an underactive thyroid.

  • Adequate: about 100 to 199 µg/L when not pregnant, 150 to 249 µg/L in pregnancy, and 100 µg/L or above while breastfeeding. A minimum of 150 µg/L is the target in pregnancy.

  • Too much: persistently high intake (above about 300 µg/L when not pregnant, or 500 µg/L and above in pregnancy) can also trigger thyroid problems, including an overactive thyroid.

A spot urine sample is only a rough guide for an individual. A 24-hour urine collection is more accurate but rarely practical. A thyroid blood test (TSH) shows how your thyroid is coping, not how much iodine you take in.

Know the signs

Too little iodine can look like an underactive thyroid: tiredness, feeling cold, dry skin, constipation, weight gain, or swelling at the front of the neck (a goitre). Too much can look like an overactive one: palpitations, weight loss, anxiety and feeling hot. See your GP if you notice these.

How much do you need?

Under the Australian and NZ Nutrient Reference Values:

  • Reproductive-age women: 150 µg per day (the RDI)

  • Pregnancy: 220 µg per day

  • Breastfeeding: 270 µg per day

  • Upper limit: 1100 µg per day

Why does the requirement jump? In pregnancy your baby relies on your thyroid hormone for brain development, and while breastfeeding, iodine passes into your milk.

Should you supplement?

The NZ Ministry of Health recommends 150 µg of iodine daily throughout pregnancy and breastfeeding, and many clinicians suggest starting when you are trying to conceive. Because of that U-shaped curve, stick to the recommended dose, avoid kelp supplements, and check with your GP first if you have a thyroid condition.

Food first: build an iodine-smart plate

Good sources are iodised salt, fortified bread, dairy, seafood, eggs and seaweed. Keep seaweed portions modest, as iodine content varies hugely, especially in kelp.

Try these pairings:

  • Baked cod with steamed broccoli: a reliable seafood iodine source. Cooking cruciferous vegetables like broccoli reduces their goitrogenic effect compared with eating them raw.

  • Boiled egg and a seaweed snack: two iodine sources combined into a simple, repeatable daily habit.

  • Tuna and egg sandwich on fortified bread: seafood, egg and bread, all iodine contributors in one lunch.

The takeaway

Plan ahead rather than react. Use iodised salt, eat fortified bread, dairy, eggs and seafood regularly, and ask your GP or nutritionist about a supplement if you are trying to conceive, pregnant or breastfeeding.