The thyroid is an endocrine (hormone-secreting) gland which sits at the base of the throat and regulates many metabolic processes, including body temperature and the metabolic rate.
In order to produce the thyroid hormones T4 and T3, the thyroid needs a reliable source of two key trace elements: iodine and selenium. Other nutrients (such as iron and zinc) are also required for optimal thyroid function, but in this post I'll focus on iodine and selenium, as they are directly involved in the synthesis of thyroid hormones, and without dietary planning, they are especially at risk of falling short in plant-based diets.
Iodine deficiency disorders are a major public health concern in Australia, as around half (43%) of the population don’t meet the RDI for iodine. The amount of iodine and selenium in plant foods depends on the soil concentration, and for iodine in particular, the soil in many areas of Australia where food is grown is notoriously low.
Dietary surveys show that for Australians, the main sources of these two nutrients are seafood, milk and eggs for iodine, and seafood, poultry and eggs for selenium, so where are we going to get our iodine and selenium on a plant-based diet?
The only sources of iodine on a plant-based diet are iodised salt, seaweeds (also known as sea vegetables) or a daily multivitamin (which generally contain 100% of the RDI).
Bread is also a source of iodine as bread manufacturers are required by law to use iodised salt (in place of regular salt) as an ingredient. 100g of bread (3 thin slices) contains roughly a third of the RDI for iodine. (NB/ organic breads are exempt from this legislation, which means if you’re an organics consumer, you won’t be getting a significant amount of iodine from bread.)
Iodised salt used to be an efficient way of getting iodine to the masses, but most people have ditched it in favour of fancier boutique salts - such as pink salt flakes or Himalayan salt - which don’t contain appreciable amounts of iodine. If you don’t add salt to your food, I am certainly not recommending that you start doing so just to get the iodine - this is where a multivitamin containing 100% of the RDI for iodine would be appropriate. However, if you are a salt-user, switching to iodised salt can help you to get more iodine into your diet.
Seaweed/ Sea Vegetables
As a general rule, brown seaweeds, such as arame and wakame tend to be higher in iodine than red seaweeds such as nori and dulse. If you regularly eat brown seaweeds, I would advise limiting them to once or twice per week and only consuming them in small quantities, as excess iodine can be toxic to the thyroid gland.
I have previously recommended using dulse flakes daily as a source of iodine, but have since calculated that you need quite a decent sprinkle on a meal to meet the RDI, so I now recommend either nori (2 sheets per day) or a multivitamin (containing 100% of the RDI for iodine) daily.
Seaweeds to avoid
Hijiki seaweed has been found to be high in naturally-occurring inorganic arsenic, and is not safe for human consumption (FSANZ, 2004).
Dried kelp is also not a good idea as it is extremely high in iodine. The use of powdered kelp in Bonsoy (a brand of soy milk) between 2004 and 2009 resulted in extremely high levels of iodine in the milk and was responsible for ‘poisoning’ and causing extensive health problems in hundreds of consumers (who later won a massive class action law suit against the parent company).
Is there any harm in having too much iodine?
Although it’s important to ensure you’re getting enough iodine it’s important to note that the safety margin for iodine is quite narrow – the upper safe limit is just 1,100mcg, which is only 7 times the RDI of 150mcg for adults. Although high intakes of iodine don’t cause thyroid problems, they can unmask hyperthyroidism (an overactive thyroid gland) and increase the severity of the condition. For this reason, I advise against kelp supplements for iodine, which have been shown to have varying levels of iodine and can be contaminated with heavy metals such as arsenic.
The role of selenium in the functioning of the thyroid gland is less well understand than for iodine, but it is known that selenium is required by the an enzyme which convert the less active thyroid hormone (T4) to the more active form (T3), and is also a key component of an antioxidant enzyme glutathione peroxidase which function inside the cells of the thyroid gland.
As with iodine, the selenium content of plant foods depends on the soil the food is grown in, and is often low in areas low in iodine. Luckily, there are certain plants are particularly good at hoarding selenium (‘seleniferous’ plants). Brazil nuts fall into this category, as they are a particularly reliable source of selenium. Just 2 brazil nuts contains 74mcg, which is 100% of the RDI for selenium (70mcg for men and women).
Taking a daily multivitamin is an easy and reliable way to ensure you're getting the RDI for both iodine (150mcg) and selenium (70mcg), so if you're already one then there's no need to be concerned about food sources of these nutrients.
For further reading on iodine levels in seaweeds: