Iodine after the age of 40

After the age of 40 the thyroid gland changes: nodules, autoimmune thyroiditis and hypothyroidism become more frequent. Because of this, the familiar advice to 'take iodine' becomes less clear-cut. The editors examine whether the need for iodine increases with age, when it is beneficial and when it can provoke problems.

Does the need for iodine change with age

Official iodine intake norms do not change with age. For adults of any age the US Institute of Medicine recommends 150 mcg per day, and EFSA sets adequate intake at the same level. The tolerable upper intake level is 1100 mcg according to the IOM and 600 mcg according to EFSA. In other words, the 'age-related' feature after 40 is not the norm figure, but the state of the thyroid gland into which this iodine enters.

Over the years, structural changes accumulate in the gland. The frequency of nodules detected on ultrasound increases with each decade of life, and in regions with long-standing iodine deficiency, to which Ukraine historically belongs, nodular and multinodular goiter is especially common among middle-aged and older people.

In parallel, the frequency of autoimmune thyroiditis and hypothyroidism increases, especially in women. Subclinical hypothyroidism, when TSH is elevated but T4 is still normal, is detected quite often in older age groups. These conditions are rarely associated with a lack of iodine in countries where salt is iodized, so taking iodine on your own does not fix them.

So, after the age of 40, the question is not 'how much iodine to take' but 'is it safe for me to add iodine given my thyroid gland'. For a person with a healthy gland the answer is simple; for a person with nodules it is much more complicated.

Nodular goiter and the risk of iodine-induced hyperthyroidism

The most important age-related risk is iodine-induced hyperthyroidism, known as the Jod-Basedow phenomenon. It occurs when there are autonomous nodules in the gland that produce hormones independently of the pituitary signal. As long as iodine is lacking, such nodules 'starve' and function remains normal. A sharp increase in iodine intake gives them raw material, and hormone levels rise.

Epidemiological data describe this pattern at the population level: after the launch of salt iodization programs in regions with previous deficiency, the frequency of hyperthyroidism temporarily rose, primarily among older people with multinodular goiter. Later, as the generation with long-standing deficiency is replaced, the frequency decreases.

For a person after 40, hyperthyroidism is not just unpleasant symptoms such as palpitations, tremor and weight loss. An excess of thyroid hormones increases the risk of atrial fibrillation and contributes to loss of bone mass, which is especially dangerous in women during menopause.

Sources of excess iodine are most often not ordinary multivitamins, but supplements with seaweed, 'natural' thyroid preparations, iodine-containing radiographic contrast agents and the antiarrhythmic drug amiodarone. People with nodules should tell their doctor about all such sources.

Reference values and upper limits: what official sources say

For convenience we have summarized the key benchmarks in a table. Note the difference between the upper limits of the IOM and EFSA: European specialists chose a more cautious threshold, in particular because of data on the sensitivity of people with thyroid diseases.

It is important to understand that the upper limit applies to healthy people. For people with autoimmune thyroiditis or autonomous nodules, functional disturbances can occur even at lower intakes. Therefore a 'safe' dose after the age of 40 is individual.

It is also worth counting all sources together. A multivitamin complex, a separate iodine preparation, a 'thyroid complex' and kelp capsules taken at the same time can imperceptibly bring the total intake close to the upper limit or exceed it.

At the same time, people who eat a varied diet and use iodized salt usually get enough iodine without any supplements. For them, additional intake is unlikely to provide any benefit.

Iodine, metabolism and training after 40

Among people who start training or losing weight actively after the age of 40, there is a popular belief that a 'slowed metabolism' can be sped up with iodine. This is a myth. If the thyroid gland works normally, additional iodine does not raise hormone levels and does not speed up energy expenditure. The decrease in energy expenditure with age is related primarily to the loss of muscle mass and reduced activity.

Real cases when a trained person over 40 should consider iodine:

• long-term use of only non-iodized salt or almost complete rejection of salt;
• a plant-based diet without seafood and dairy products;
• very intense training with profuse sweating against the background of a poor diet.

Even in these situations it is sensible to start not with a supplement but with checking thyroid function and adjusting the diet. Sea fish, dairy products, eggs and iodized salt in moderate amounts usually cover the need.

If a supplement is needed, for a person without thyroid diseases 100-150 mcg of potassium iodide per day is usually sufficient, often as part of a multivitamin complex. There is no need to duplicate iodine from several products at once.

Which examinations are appropriate and when to see a doctor

After the age of 40 it is sensible to periodically check TSH, especially for women, people with a family history of thyroid diseases, and those who feel unexplained fatigue, changes in weight, palpitations or intolerance to cold or heat. If necessary, the doctor will add free T4, TPO antibodies and an ultrasound of the gland.

An iodine test in a single urine sample is of little use for an individual: the result depends heavily on what the person ate over the last day. This indicator was created for assessing populations, so there is no point in taking it 'to select a dose'.

You should see a doctor immediately if, after starting to take iodine or seaweed, you develop palpitations, tremor, insomnia, sweating or noticeable weight loss. These may be signs of hyperthyroidism, and for a person over 40 such conditions are dangerous because of the strain on the heart.

It is also important to warn your doctor about taking iodine-containing supplements before contrast studies, before treatment with radioactive iodine and when amiodarone is prescribed. An excess of iodine can change the results and the effectiveness of treatment.

Editors' conclusions

The iodine norm after the age of 40 is the same as at a younger age - 150 mcg per day. What changes is something else: with age, nodules and autoimmune changes in the thyroid gland become more frequent, and then an excess of iodine can cause both hyperthyroidism and hypothyroidism.

For most people, iodized salt and a varied diet are sufficient. A supplement is appropriate only with a poor diet and a healthy gland, while large doses, seaweed and attempts to 'rev up the metabolism' with iodine are an unjustified risk.

We also recommend reading our articles on iodine for women, on selenium after the age of 40 and on tests for monitoring thyroid function.

References

1. Institute of Medicine (US) Panel on Micronutrients. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington (DC): National Academies Press; 2001.
2. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for iodine. EFSA J. 2014;12(5):3660.
3. Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol. 2014;10(3):136–142.
4. Laurberg P, Cerqueira C, Ovesen L, et al. Iodine intake as a determinant of thyroid disorders in populations. Best Pract Res Clin Endocrinol Metab. 2010;24(1):13–27.
5. Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550–1562.
6. Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol. 2015;3(4):286–295.
7. World Health Organization. Assessment of iodine deficiency disorders and monitoring their elimination: a guide for programme managers. 3rd ed. Geneva: WHO; 2007.