Iodine test: what the result shows

Iodine is a trace element that the body does not make itself, so it has to be obtained from food (seafood, iodised salt, milk). The thyroid uses iodine to make the hormones thyroxine (T4) and triiodothyronine (T3) — these hormones regulate metabolism, heart rhythm, body temperature and, especially during pregnancy, the development of the baby's brain. The amount of iodine in blood or urine reflects the iodine taken in with food recently, so the result can vary depending on the last meal or on supplements. For this reason an iodine test is most often used not to diagnose an individual person, but together with other thyroid markers (TSH, free T4, free T3) to assess whether the body is getting enough iodine. It is also used in population-level studies to assess how common iodine deficiency is in a region. Long-term iodine deficiency or excess can affect the way the thyroid works, so the result is always worth discussing with a doctor and alongside other thyroid tests.

• When a doctor suspects an enlarged thyroid (a goitre) and wants to assess whether iodine deficiency could be the cause
• When looking into the causes of hypothyroidism or hyperthyroidism, together with TSH, free T4 and free T3 tests
• To monitor whether iodine intake is sufficient during pregnancy or breastfeeding, when the need for iodine increases
• When the diet does not include iodised salt, seafood or dairy products (for example on a strict plant-based diet)
• In population or epidemiological studies, to assess how common iodine deficiency is in a region

A raised iodine level is most often linked to taking in too much iodine from food, from supplements or from substances that contain iodine. According to MedlinePlus, a very large amount of iodine can reduce thyroid activity and, together with some medicines that suppress the thyroid, cause hypothyroidism. The result is worth assessing together with TSH and free T4.

What can raise iodine

• A large intake of iodine from food or in supplements — seaweed, seaweed products, iodine supplements
• A recent procedure with an iodine-containing contrast agent (for example imaging with contrast)
• Some medicines prescribed for heart rhythm disorders, which contain iodine
• Areas where the drinking water or the soil is naturally high in iodine

What is usually assessed alongside a raised result

• TSH and free T4 tests — to assess whether the raised iodine level has affected the way the thyroid works
• A conversation with the doctor about supplements, seaweed products or contrast agents used recently
• If you take medicines that may contain iodine — discussing with your doctor whether thyroid function needs to be monitored
• Repeating the test once time has passed after a possible temporary source of the effect has been removed

A low iodine level most often shows that not enough iodine is being taken in with food. According to MedlinePlus, long-term iodine deficiency can cause a goitre (an enlarged thyroid) or hypothyroidism, and in severe cases in children — serious developmental disorders. The result is worth assessing together with TSH and free T4.

What can lower iodine

• Not enough iodine in the diet — no iodised salt, seafood or dairy products are used
• A strict plant-based diet without iodine sources or supplements
• An increased need for iodine during pregnancy or breastfeeding that the diet does not cover
• Living in a region where the soil and the water are naturally poor in iodine

Signs that can accompany a low iodine level

• An enlarged thyroid (a goitre)
• Tiredness and not tolerating cold — typical signs of hypothyroidism
• In children — growth and developmental disorders, if the deficiency is long-term

What is usually assessed alongside a low result

• TSH and free T4 tests — to assess whether the iodine deficiency has affected the way the thyroid works
• A review of the diet with a doctor or a dietitian — the use of iodised salt and seafood
• For pregnant and breastfeeding women — discussing the need for iodine with a doctor separately
• Repeating the test to assess the change after the diet has been adjusted