The thyroid uses iodine as its main raw material and so this radioactive form concentrates in the colloid (hormone forming) tissue in the thyroid gland.
I 131 concentrates in the cells and its radioactivity destroys them. Depending on how much is given initially depends of course on the severity of cellular damage. The amount given is calculated by body weight and the presumed severity of the over-activity of the thyroid forming cells.
There are three possible scenarios. One is that the calculation was right. The amount of thyroid tissue left is just right to produce the right level of hormones in the bloodstream. Of course, the cells may later partly recover, and then it may have to be done all over again or further damage and loss of function may occur and the thyroid as a whole may become underactive.
The second scenario is that the patient continues to have an overactive thyroid in spite of treatment, and a further dose of radioactive iodine – or doses – may have to be given at once.
The third scenario is a good deal more common. Overkill becomes evident in a few days, and thyroid hormone in the bloodstream falls pretty quickly. Very soon thyroid replacement (usually thyroxine) becomes necessary.
Iodine, a relatively rare element, is a member of a class of chemical elements known as halogens and has many important roles in the body. However, it can be crowded out by other toxic halogens which compete with each other for receptor and binding sites in the body.1,2 The other halogens are:3
Some people get confused about iodine and iodide. Iodide is the state of iodine when it bonds with another element, such as potassium. In this form, iodine can be ingested or applied topically.
Iodine and iodide are actually different expressions of the same element. Iodides represent a safe form of iodine for ingestion.5 When you purchase iodine from the local drug store, it’s usually an iodide solution. This combination allows the body to absorb and use it safely. Dietary iodine also occurs naturally as an iodide, such as potassium iodide or sodium iodide (i.e. as in salt).
Iodine-containing compounds are extracted from the ashes of burnt seaweed and found in salty oil well brines, Chilean saltpetre, seawater and solid rocks that form when seawater evaporates. Seaweed concentrates large amounts of iodine from ocean water and it is also found in some types of soil.6
Iodide plays an important role in regulating the activity of the thyroid gland. It is found in every cell of the body and is stored in particularly high concentrations in the thyroid gland, whilst large amounts are also stored in the salivary glands, the cerebrospinal fluid, the brain, the mucus membrane layer of the stomach, the choroid plexus (four sets of branching vessels in the brain where cerebrospinal fluid is produced), the breasts, the ovaries, and the ciliary body of the eye.7
Iodine deficiency is associated with goitre and hypothyroidism; in England, this is sometimes known as Derbyshire Neck. Babies born with severe thyroid function deficiency which is left untreated for several months may be at risk of growth failure – thyroid hormones influence growth from birth up to age 20 – and permanent mental retardation (cretinism).8
The Society for Endocrinology stated:
“Young women of child-bearing age are the most susceptible to the adverse effects of iodine-deficiency and even mild deficiency may have an impact on the developing brain of foetuses and young children. It can also cause goitre. According to the World Health Organisation, iodine-deficient communities have IQs up to 13.5 points lower than similar but iodine-sufficient communities.” 9
A 2011 study of the iodine status of 14 to 15-year-old school girls in the UK concluded that the UK, like Australia and the USA, is now iodine-deficient.10,11 Young women of childbearing age are the group most susceptible to the adverse effects of iodine deficiency. Nearly 70% of the samples were below 100ug/L and 18% of samples showed very low iodine levels below 50μg/L. The researchers considered this finding to be of huge public health importance since mild iodine deficiency impairs cognition in children and severe iodine deficiency reduces IQ by 10 to 15 points.
Dr Mark Vanderpump, one of the authors of the research article on iodine states, ‘One cup of milk contains about half the amount of iodine needed per day. A possible explanation for our findings may be that teenage girls are drinking less milk and are less likely to have milk and cereal for breakfast. The solution could be to add iodine to the salt used in products such as bread as has been done in for example Denmark.’ The same could be said of young children who are not now given free milk at school and adults, who think that drinking milk is fattening.
In response to the Lancet article, the Department of Health’s Scientific Advisory Committee on Nutrition asked for a review of the current evidence10 that some degree of iodine deficiency might affect some groups within the UK population. They agreed that “the issue of iodine intakes is of considerable public health significance, although the Committee is cautious in drawing conclusions on current evidence due to the limitations of the available data,”12
Iodine has a number of roles to play in the body13,14
The World Health Organisation, The United Nations Children’s Fund (UNICEF), and the International Council for the Control of Iodine Deficiency Disorders (ICCIDD) give the following recommended daily intake of iodine.20
| Life stage | Iodine RDI (mcg)* |
|---|---|
| Preschool children (0-59 months) | 90 µg |
| Schoolchildren (6 to 12 years) | 120 µg |
| Adults (above 12 years) | 150 µg |
| Pregnant and lactating women. | 250 µg |
For some people, iodine or iodine-containing products may worsen autoimmune thyroid problems such as Hashimoto’s and Graves’ disease, and cause enlargement of the thyroid (goitre).21 On the other hand, some healthcare professionals believe that a high iodine intake, such as seen in Japanese women, can largely prevent breast cancer.2
If you are thinking of taking iodine supplements, consider having your levels measured to see if you have an actual deficiency. Once you start supplementing, watch out for any symptoms that develop.
You need to look carefully at any medications you take as iodine is found in a number of these.
Also, studies have shown that exposure to iodinated contrast media used for X-ray investigations such as angiograms, CT scans and MRI scans is associated with the subsequent development of thyroid disease. 22,23
Be aware, too, that some people have adverse reactions to iodine.24,25,26
| Anti-arrythmic drugs | Amiodarone, Cordarone, Pacerone |
| Medicated douches | Betadine, Massengil, Povidone |
| Antiparasitics | Iodoquinol, Yodoxin, Vytone |
| Iodine topical ointments | Povidone, Betadine |
| X-ray dyes | CT scans, intravenous pyelogram (IVP), arteriograms, angiograms, myelograms |
| Intravenous pyelograms (IVP’s) | radiological procedure used to visualize abnormalities of the urinary system, including the kidneys, ureters, and bladder. |
| Potassium iodine | Lugol’s solution |
| Potassium Iodide | |
| Kelp | Seaweed |
Iodine deficiency can be tested for in various ways:
Enough iodine tincture (a mixture of iodine and potassium iodide) is placed on a cotton ball to paint a two-inch circle in a soft place of your body such as your inner thigh or upper arm. If the painted area has disappeared after an hour, your body is seriously deficient in iodine. If the painted area is still there after four hours, your body is not deficient in iodine. However, there is no evidence that this is a reliable test on humans.27
A urine iodine test is most often used in research studies looking into iodine deficiency and is accepted as a good marker of very recent dietary iodine intake20 although there are some controversies over this test.28
Nutritional supplements and medication may affect the result (they may contain iodine) so you should check with your doctor whether they should be discontinued before the test. Non-essential supplements should be discontinued 48 hours before the test.
It is also essential that you don’t drink an excessive amount of fluids for 24 hours before the test as this may affect the result.29
In an iodine loading test, iodine sufficiency is determined by comparing the amount of Iodine taken to the amount excreted in the urine.30 After ingesting a specified oral dose of iodine, urine is collected for 24 hours and iodide levels are then measured. Because taking iodine is not good for certain people, you should always discuss having this test with your healthcare practitioner.
If the body has sufficient iodine, most of the ingested iodine will be excreted in the urine. The more deficient in iodine the body is, the less iodine will be excreted in the urine. Whole body iodine sufficiency is reached when 90% or more of the ingested amount is excreted in the urine.
The 24 Hour iodine loading test is not available on the NHS in the UK but is available via Regenerus Laboratories. However, there is some evidence that this test does not provide a realistic assessment of an individual’s whole body iodine sufficiency or deficiency due to flaws in the methodology and the use of an arbitrary excretion cut-off point.31,32
Iodine levels can also be measured by means of a blood test. However, some doctors feel that this isn’t as good as a urinary test 33 and urine testing seems to be the favoured test for research. If your doctor is unable to do an iodine blood test, you can obtain this from Medichecks.
It is quite likely that your NHS GP will not do a test because the Department of Health advisers state that you should be able to get all the iodine you need by eating a varied and balanced diet.34
If your doctor has confirmed that you are deficient, there are several options but you do need to be careful because too much supplementation can cause problems. You could try increasing your iodine levels by eating foods that contain iodine before you try supplementation.
Iodine is found in the following foods 35:
| Food | Portion | Average iodine/portion (mcg) (actual iodine content will vary) |
| Cow’s milk | 200ml | 50-80** |
| Organic cow’s milk | 200ml | 30-65** |
| Yoghurt | 150g | 50-100** |
| Eggs | 1 egg (50g) | 25 |
| Cheese | 40g | 15 |
| Haddock | 120g | 390 |
| Cod | 120g | 230 |
| Place | 130g | 30 |
| Salmon Fillet | 100g | 14 |
| Canned Tuna | 100g | 12 |
| Prawns | 60g | 6 |
| Scampi | 170g | 160 |
| Meat | 100g | 10 |
| Poultry | 100g | 10 |
| Nuts | 25g | 5 |
| Bread | 1 slice (36g) | 5 |
| Fruit and veg | 1 portion (80g) | 3 |
**Depending on the season, higher value in winter
It’s also found in iodized salt, sea salt, seaweed, kelp, foods that contain carrageen, agar-agar, algin or alginate, many prepared and/or cured meats (ham, bacon, sausage, corned beef), dried fruit, margarine, canned vegetables, commercial bakery products, chocolate, molasses, soy products, any vitamins or supplements that contain iodine, FD&C red dye #3, cereals – wheat etc, (depending on how much iodine was in the soil where they were grown).
However, if this is not possible for you to eat these foods or you remain deficient there are iodine supplements available:
The Department of Health advises that “you should be able to get all the iodine you need by eating a varied and balanced diet. If you take iodine supplements, don’t take too much as this could be harmful. Taking 0.5mg or less a day of iodine supplements is unlikely to cause any harm.”
There is a lot of controversy over the use of iodine.36 You need to research well before you consider which action to take. Talk to your doctor if you are not sure what dosage to take for deficiency.
Read more in-depth information about the thyroid and iodine.
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Dated updated: 16/04/2021 (V1.3) Review date: 11/10/21
Dated updated: 25/08/2021 (V1.4)
Review date: 11/10/21
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