Iodine and Thyroid Function
Iodine is vital for good thyroid function, which in turn is
essential for health. Iodine deficiency during pregnancy and early
infancy can result in cretinism (irreversible mental retardation and
severe motor impairments). In adults low iodine intake (or very high
intakes) can cause hypothyroidism. Hypothyroidism can manifest as
low energy levels, dry or scaly or yellowish skin, tingling and
numbness in extremities, weight gain, forgetfulness, personality
changes, depression, anaemia, and prolonged and heavy periods in
women. Goiter, an enlarged thyroid gland visible between the Adam's
apple and the collar bone, is often present. Hypothyroidism can also
cause carpal tunnel syndrome and Raynaud's phenomenon.
Hypothyroidism can lead to significant increases in cholesterol
levels and homocysteine levels is implicated in about 10% of cases
of high cholesterol levels. Correcting hypothyroidism can lead to a
30% drop in cholesterol and homocysteine levels.
An iodine intake of less than 20 micro grams (µg) per day is
considered severe deficiency, 20 -50 µg/day is considered moderate
deficiency and 50-100 µg/day is considered mild deficiency.
Iodine is typically undesirably low (about 50 micrograms/day
compared to a recommended level of about 150 micrograms per day) in
UK vegan diets unless supplements, iodine rich seaweeds or foods
containing such seaweeds (e.g. Vecon) are consumed. The low iodine
levcls in many plant foods reflects the low iodine levels in the UK
soil, due in part to the recent ice-age. About half the iodine
consumption in the UK comes from dairy products. In the US iodised
salt is widely used and some other foods are fortified with iodine.
In Canada all table salt is iodized. The UK has no iodine
fortification strategy for plant foods or salt.
Low zinc intakes exacerbate the effect of low iodine intake. Some
otherwise healthful foods contain goitrogens - substances which can
interfere with iodine uptake or hormone release from the thyroid
gland. These foods are generally only a concern if iodine intake is
low. Consumption of brassicas, such as cabbage, Brussels sprouts,
broccoli and cauliflower, increase the requirements for iodine,
especially if consumed raw. Soy beans, raw flaxseed, cassava (used
in tapioca), sweet potatoes, lima beans, maize and millet also
increase the requirements for iodine.
It is important not to over-consume iodine as it has a relatively
narrow range of intakes that reliably support good thyroid function
(about 100 to 300 micrograms per day). Someone consuming large
amounts of iodised salt or seaweeds could readily overdo it.
Excessive iodine has a complex disruptive effect on the thyroid and
may cause either hypothyroidism or hyperthyroidism, in susceptible
individuals, as well as increasing the risk of thyroid cancer.
Hyperthyroidism may also occur, particularly in elderly people, due
to long term slight iodine deficiency as this may result in
additional nodules on the thyroid.
Hyperthyroidism may manifest as an enlarged thyroid (goiter),
heart rate irregularities, tremor, sweating, palpitations,
nervousness and increased activity and eye abnormalities. Some
individuals deliberately take kelp to try to lose weight by over
stimulating the thyroid. This is a dangerous practice.
Subclinical hypothyroidism, with raised thyroid stimulating
hormone (TSH) levels but mild or absent overt symptoms, has been
found to be more common among vegans than the general population.
Most vegans have low iodine intakes but a significant minority
consume excessive amounts of iodine from seaweed, particularly kelp.
Both low and excessively high iodine intakes in vegans have been
linked to elevated TSH levels.
The key to good thyroid function is adequate, but not excessive
iodine intake. Intakes in the range 100-300 micrograms per day are
desirable, though intakes up to 500 micrograms per day are probably
not harmful. If taking supplements go for about 100-150 micrograms
per day, to give a total intake of 150-200 micrograms per day. The
supplements supplied by The Vegan Society contain an average of
about 150 micrograms, so four a week provides about the right
amount. Most supplements contain higher levels and should be
restricted to two a week.
If using seaweeds as an iodine source it is best to use seaweeds
that have been found to have a fairly consistent iodine content,
such as kelp (kombu) or hijiki. Consumption of more than 100g/year
(by dried weight) of most seaweeds carries a significant risk of
thyroid disorder due to iodine intakes in excess of 1000 micrograms
Nori is low in iodine and several sheets a day can be eaten
without any concern about excess iodine. Frequent addition of small
amounts of powdered or crumbled seaweed to stews or curries while
cooking, or to other foods as a condiment, is an excellent way to
provide adequate iodine (in the absence of other supplementation)
and is a healthful practice for vegans. 100g of dried hijiki or 15g
of dried kombu or kelp in a convenient container in the kitchen
provides one year's supply for one person.
Most vegans know that B12 deficiency can cause neurological
complications and tingling sensations or numbness. B 12 deficiency
is also a common cause of elevated homocysteine levels in vegans. It
should be noted that hypothyroidism (myxedema) can also cause nerve
damage, tingling sensations and elevated homocysteine and should be
considered as an alternative diagnosis for these symptoms.
Thyroid function can be readily tested by doctors based on a
blood sample and measurement of thyroid related hormone levels.
Information courtesy of Stephen Walsh