Urinary iodine (24-hour urine collection) – laboratory test

Description

Why urinary iodine is tested

About 90% of ingested iodine is excreted in urine, so its concentration in urine is the best indicator of the body’s current iodine intake. Iodine is essential for thyroid hormone production; deficiency leads to goitre and hypothyroidism, and during pregnancy it disrupts the baby’s brain development. Excess iodine (supplements, algae, kelp, contrast) can trigger hyperthyroidism or worsen autoimmune thyroid disease. A 24-hour collection averages out day-to-day fluctuations and gives a more accurate picture than a single sample.

When to consider this test

• planning a pregnancy, pregnancy and breastfeeding (requirements increase to approx. 250 µg/day),
• goitre or thyroid nodules, hypothyroidism without antibodies, abnormal TSH in someone on a salt-free diet,
• a vegan diet, restrictive elimination diets, avoiding fish and dairy products, limiting salt (table salt in Poland is iodised),
• suspected excess intake: iodine supplements, algae, kelp, Lugol’s solution, iodine-containing medicines (amiodarone),
• monitoring iodine supplementation prescribed by a doctor.

How to prepare and collect your urine

• For 3–5 days before collection, do not take iodine supplements, algae or kelp; do not eat large amounts of sea fish or seafood. Otherwise, do not change your usual diet, as the aim is to assess your everyday intake.
• After iodinated contrast (CT scan, coronary angiography), wait several weeks.
• A 2–3 l container (from a pharmacy or collection point; ask in advance) and a small sample container.
• Collection: in the morning, pass your first urine into the toilet and note the time; collect all subsequent urine over 24 hours in the large container, kept in the fridge; finish with the first morning urine the following day. Mix, measure the volume, pour off 50–100 ml and label with: first name, surname, PESEL (Polish personal identification number), total volume (e.g. 2450 ml), DZM (24-hour urine collection).
• Missing one urine sample invalidates the test.

How to interpret the result

The laboratory reports the concentration in µg/l and often the daily excretion in µg/24 h. WHO thresholds are established for population medians, but doctors also use them as a guide for individual patients:

The result depends on your diet over the preceding days and the amount of fluid you have drunk; when deciding on supplementation, your doctor assesses it alongside TSH, antibodies and thyroid ultrasound (USG). Daily requirements according to WHO: approx. 150 µg for adults, approx. 250 µg during pregnancy and breastfeeding.

Other tests to consider

TSH, FT4, anty-TPO, and, if excess iodine is suspected, also FT3; during pregnancy, a thyroid test package. More about iodine and the thyroid: Badania-Medyczne.pl.

Sources: ALAB laboratoria, Urinary iodine, 24-hour collection (collection instructions, interfering factors, requirements according to WHO); WHO, Nutrition Landscape Information System: Iodine deficiency (urinary iodine concentration thresholds for the general population and pregnant women). This content is for information only and does not replace a medical consultation.