Iodine Test
A urine-based test that assesses iodine levels to help understand thyroid support, metabolism and overall nutrient status. Useful when symptoms suggest low iodine intake or when thyroid-related concerns persist despite normal blood tests.
How iodine status is assessed
The most common method for assessing iodine status is measuring urinary iodine concentration (UIC) in a spot (single) urine sample. Approximately 90 percent of dietary iodine is excreted in the urine, usually within 24 to 48 hours, so urinary iodine reflects recent iodine intake quite accurately.
However, iodine intake varies significantly from day to day depending on what you eat, so a single spot urine sample has high day-to-day variability and is not considered reliable for assessing an individual's iodine status. It has been estimated that around 10 separate urine samples would be needed to establish an individual's true iodine status with reasonable accuracy. Therefore some laboratories offer 24-hour urine iodine collections, which are more reliable than spot samples but are less practical and still subject to variation based on recent diet.
Different labs use slightly different approaches, but the goal is the same: to understand whether iodine intake may be contributing to symptoms related to thyroid health, energy, temperature regulation or metabolism.
What is involved in an iodine test (sample and prep)
The process of doing an iodine test is straightforward. You collect, at home, either a single urine sample,or a series of samples over 24 hours depending on the panel. There is no blood draw and minimal preparation is needed.
Who might consider checking iodine levels?
Iodine testing may be considered if you:
Are pregnant or planning pregnancy and are concerned about iodine intake, particularly if you avoid dairy, fish and iodised salt.
Follow a vegan or restricted diet and are not sure whether you are getting enough iodine.
Have symptoms of thyroid dysfunction (for example, fatigue, weight changes, feeling cold, hair loss, dry skin) and want to understand whether iodine status may be playing a role, particularly if thyroid function tests are borderline or normal.
Live in an area known to have low iodine intake (although most of the UK now has adequate iodine intake due to iodine in dairy products).
Have a history of thyroid nodules, goitre (enlarged thyroid gland) or autoimmune thyroid disease and want to monitor iodine status.
Are taking iodine supplements or eating large amounts of iodine-rich foods (for example, seaweed) and want to check that intake is not excessive.
However, it is important to remember that a single urine iodine test is not a reliable way to diagnose iodine deficiency or excess in an individual, and results should always be interpreted alongside thyroid function tests, symptoms, diet and medical history.
Why iodine testing is not routinely offered by your GP
Iodine testing is not part of routine NHS screening because iodine deficiency is relatively uncommon in the UK, particularly since the widespread use of iodised salt and the iodine content of milk and dairy products. Thyroid function tests (TSH, free T4, free T3) are considered more reliable and clinically useful for diagnosing thyroid dysfunction, which is what GPs need to identify in order to prescribe treatment or refer for specialist care.
But importantly, it's not feasible for GPs to test for every nutrient affecting thyroid health. There are many possible contributors (iodine, selenium, iron, B12, vitamin D, stress, inflammation). The NHS cannot routinely test all of these for everyone without a clear clinical indication. Where iodine deficiency is suspected (for example, in pregnancy or in someone with a goitre and low thyroid function), GPs may arrange iodine testing through the NHS, but this is not common.
Ultimately, NHS resources focus on tests that change medical treatment. Iodine levels do not usually alter prescribed medication in the early stages. For this reason, they fall outside routine NHS testing, even though they can be very informative in a nutritional context.
How iodine findings are interpreted alongside thyroid function and diet
If an iodine test is done, the results should always be viewed in the context of:
Thyroid function tests (TSH, free T4, free T3 and, where relevant, thyroid antibodies).
Your diet, particularly intake of dairy products, fish, eggs, seaweed and iodised salt.
Symptoms, which may include fatigue, weight changes, feeling cold, hair loss, dry skin, constipation, low mood or, in the case of excess iodine, palpitations, anxiety or worsening of thyroid autoimmunity.
Pregnancy or breastfeeding status, as iodine requirements are higher during these times.
Use of iodine supplements or iodine-containing medications (for example, amiodarone, a heart medication).
Low urinary iodine alongside symptoms and low or borderline thyroid function may suggest that iodine intake is inadequate and that increasing dietary iodine (through dairy, fish or, where appropriate, a supplement) could be helpful.
High urinary iodine, particularly if accompanied by thyroid dysfunction or symptoms of hyperthyroidism (for example, anxiety, palpitations, weight loss), may suggest excessive iodine intake, which can occur with high seaweed consumption or iodine supplementation and can sometimes trigger or worsen thyroid autoimmunity.
Using iodine results to adjust food choices safely
If iodine testing suggests that intake is low, the focus is usually on increasing iodine-rich foods rather than immediately starting supplements, unless there is a clear clinical need (for example, pregnancy or severe deficiency). Good dietary sources of iodine include:
Milk and dairy products (the main source of iodine in the UK diet).
Fish and seafood, particularly white fish, cod, haddock and shellfish.
Eggs (a moderate source).
Seaweed and kelp (very high in iodine, but intake should be monitored as it can provide excessive amounts).
Iodised salt (although many UK table salts are not iodised, so check the label).
If supplementation is considered, it is important to choose a dose that is appropriate and safe. The recommended daily intake of iodine is:
150 micrograms per day for adults.
220 micrograms per day for pregnant women.
290 micrograms per day for breastfeeding women.
Excessive iodine intake (above 500 to 1,000 micrograms per day on a regular basis) can be harmful and may cause or worsen thyroid dysfunction, particularly in people with existing thyroid conditions or autoimmune thyroid disease. For this reason, iodine supplementation should always be done cautiously, ideally under guidance, and seaweed supplements should be used carefully as they can contain very high and unpredictable amounts of iodine.
Thyroid and iodine-related symptoms that need prompt GP input
You should contact your GP if you experience:
Unexplained weight loss or gain.
Rapid or irregular heartbeat (palpitations).
Severe fatigue or weakness.
A lump or swelling in the neck (which may indicate a goitre or thyroid nodule).
Difficulty swallowing or breathing.
Sudden worsening of anxiety, mood or energy.
Symptoms of thyroid dysfunction (feeling very cold or very hot, significant hair loss, changes in bowel habits, changes in menstrual cycle).
Any symptom that feels severe, unusual or concerning.
If iodine testing or thyroid function tests suggest a problem, or if you are pregnant and concerned about iodine intake, your GP can arrange appropriate investigations, monitoring or referral to an endocrinologist where needed.
Let's talk
If you’re experiencing low energy, temperature changes or thyroid-related symptoms and aren’t sure whether iodine may be part of the picture, we can begin with a conversation. Sometimes this test can offer clarity; other times, simpler steps may be more appropriate.
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