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Preventing Thyroid Diseases with a
Plant-Based Diet, While Ensuring Adequate
Stewart Rose and Amanda Strombom*
Plant-Based Diets in Medicine, USA
Submission: January 23, 2020; Published: February 04, 2020
*Corresponding author: Amanda Strombom, Bellevue, Plant-Based Diets in Medicine, USA
How to cite this article: R Stewart, S Amanda. Preventing Thyroid Diseases with a Plant-Based Diet, While Ensuring Adequate Iodine Status. Glob J Oto, 2020; 21(4): 556069. DOI: 10.19080/GJO.2019.20.556069
People with thyroid disorders in the US usually have an autoimmune disease, such as Hashimoto’s thyroiditis or Grave’s disease. Those following a plant-based diet are much less susceptible to auto-immune diseases in general. They also have a lower BMI on average. Since obesity is a significant risk factor for hypothyroidism, this contributes to the lower risk of hypothyroidism among vegans. However even after controlling for BMI and potential demographic confounders, vegans have been shown to experience a 22% lower risk of hypothyroidism, and a 51% risk of hyperthyroidism. Given that they are unlikely to be consuming any seafood or dairy, iodine deficiency is a greater possibility for those following a plant-based diet. Studies show that many vegans have very adequate iodine status, but some are deficient. The median is mildly deficient, but the range can vary greatly.
Very low iodine intake can reduce thyroid hormone production even in the presence of elevated TSH levels. However, high intakes of iodine can cause some of the same symptoms as iodine deficiency—including goiter, elevated TSH levels, and hypothyroidism—because excess iodine in susceptible individuals inhibits thyroid hormone synthesis and thereby increases TSH stimulation, which can produce goiter. Laboratory tests that detect abnormal thyroid function may be more useful for diagnosing chronic iodine deficiency or excessive iodine intake and for monitoring the effects of iodine supplementation.
While the lack of iodine in the diet is the biggest risk factor for hypothyroidism worldwide, in the United States, obesity is a significant risk factor for hypothyroidism . The relation between obesity and hypothyroidism appears to have several explanations. Obesity may result in raised Thyroid Stimulating Hormone [TSH] levels, partly due to a proinflammatory milieu and other endocrine derangements . Persons with obesity are prone to develop autoimmune hypothyroidism, and even mild thyroid failure contributes to the progressive increase in body weight, which ultimately results in overt obesity . So elevated TSH levels may be both the consequence and the cause of obesity .
Vegans have a lower BMI on average  thus reducing their risk of hypothyroidism. However, the lower risk of hypothyroidism among vegans exists, even after controlling for BMI and potential demographic confounders . One study showed that following a vegan diet tended to be associated with a 22% reduced risk of hypothyroidism, although statistical significance was not quite attained .
Hyperthyroidism is a prevalent condition with many causes, of which the most common is Graves’ disease . Graves’ disease is an autoimmune disorder caused by antibodies directed against the thyrotropin receptor, resulting in excess synthesis and secretion of thyroid hormone . The third National Health and Nutrition Examination Survey [NHANES III] showed the prevalence of hyperthyroidism in the United States to be 1.3% .
While the specifics of hyperthyroidism and diet have not been extensively studied, observations by Trowel five decades ago indicated that for rural sub-Saharan Africans consuming a near-vegan diet, a number of autoimmune disorders were rare, or virtually unknown, including thyrotoxicosis and Hashimoto’s thyroiditis . Data from the Adventist Health Study showed that a vegan diet reduced the risk of hyperthyroidism by 51% while a vegetarian diet reduced the risk by 28% showing a dose response relationship .
This is to be expected since epidemiological studies have shown that vegetarians have lower risks of auto immune diseases in general. For instance, one study on Rheumatoid Arthritis [RA] showed that non-vegetarian women had a 57% increased risk of
contracting RA, and semi-vegetarians an increased risk of 16%,
when compared with vegetarian women. Non-vegetarian men
showed an increased risk of 50% and semi-vegetarian men an
increased risk of 14% . An epidemiological study found that
the risk of Crohn’s disease reduced by 70% in females and 80% in
males following a vegetarian diet .
The most common cause of thyroid disorders worldwide is
iodine deficiency, leading to goiter formation and hypothyroidism.
In iodine-replete areas, most persons with thyroid disorders have
an autoimmune disease, primarily Hashimoto’s thyroiditis .
Iodine deficiency can lead to a variety of medical problems at all
ages in the humans. This is a special concern for pregnant women.
Children of mothers having an iodine deficiency during pregnancy
may have mental retardation, deaf mutism, spasticity and short
stature. Congenital hypothyroidism due to iodine deficiency is
the most common cause of preventable mental retardation in the
world . Despite ongoing public health efforts, iodine deficiency
affects more than 2.2 billion individuals and remains the leading
cause of preventable mental retardation worldwide .
The earth’s soils contain varying amounts of iodine, which
in turn affects the iodine content of crops. In some regions of the
world, iodine-deficient soils are common, increasing the risk of
iodine deficiency among people who consume foods primarily
from those areas. Worldwide, the soil in large geographic areas
is deficient in iodine. 29% of the world’s population is estimated
to live in areas of deficiency . In the United States, before
the 1920s and the introduction of iodized salt, iodine deficiency
was common in the “goiter belt”—the Great Lakes, Appalachian,
and the Pacific Northwest, and throughout most of Canada .
Programs to iodize salt have been implemented in many countries,
significantly reducing the prevalence of iodine deficiency
In the United States table salt is available as iodized with
potassium iodide, which is used by about 50%–60% of the U.S.
population. However, the U.S. population receives only a small
percentage of its salt from table salt because 70% of dietary salt
is derived from processed food, which uses mostly non iodized
salt.  A reduction in iodine intake can also be related to
reduced salt intake for the treatment and prevention of essential
Population iodine sufficiency is defined by median urinary
iodine concentrations  (Table 1). Urinary iodine reflects
dietary iodine intake directly because people excrete more than
90% of dietary iodine in the urine.  Iodine deficiency is
diagnosed when the median iodine concentration is less than
50 μg/ml in a population . Between 1971 and 2008, urinary
iodine levels less than 50 μ g/liter among U.S. women of childbearing
age rose from 4 to 15% .
Under normal conditions, the body tightly controls thyroid
hormone concentrations via Thyroid Stimulating Hormone
[TSH] which is produced by the pituitary gland. The thyroid
gland secretes the thyroid hormones triiodothyronine [T3] and
thyroxine [T4] in response to TSH [also known as thyrotropin].
The vast majority of T4 [99.97%] and T3 [99.70%] is tightly
bound to thyroxine binding globulin [TBG] and other plasma
proteins, and it is only the unbound or free thyroid hormones that
are bioactive. Serum TSH is the most sensitive early indicator of
thyroid dysfunction . Typically, TSH secretion increases when
iodine intake falls below about 100 mcg/day . TSH increases
thyroidal iodine uptake from the blood and the production of
TSH is regulated by circulating serum free thyroxine [FT4].
Subclinical hypothyroidism is defined as elevated serum TSH
in the setting of normal FT4. In overt hypothyroidism, serum
TSH is elevated and serum FT4 is low. Serum free and total T3
concentrations usually do not decline until hypothyroidism is
quite advanced, because elevated TSH stimulates the release of T3
from the thyroid .
However, very low iodine intakes can reduce thyroid
hormone production even in the presence of elevated TSH levels.
If a person’s iodine intake falls below approximately 10–20 mcg/
day, hypothyroidism occurs , a condition that is frequently
accompanied by goiter. Moderate to severe iodine deficiency may
produce lower serum FT4 and consequent elevation of serum TSH.
If the iodine deficiency is prolonged, goiter may develop and there
may be an increase in circulating concentrations of thyroglobulin
[Tg]  This possibility should be considered when reviewing
High intakes of iodine can cause some of the same symptoms
as iodine deficiency—including goiter, elevated TSH levels, and
hypothyroidism—because excess iodine in susceptible individuals
inhibits thyroid hormone synthesis and thereby increases TSH
stimulation, which can produce goiter. [17,26] Excess iodine may
induce hyperthyroidism in some patients, resulting in elevated
FT4 and depressed TSH .
Fibrocystic breast disease is a benign condition characterized
by lumpy, painful breasts and palpable fibrosis. It commonly
affects women of reproductive age, but it can also occur during
menopause, especially in women taking hormone replacement
. Breast tissue has a high concentration of iodine, especially
during pregnancy and lactation] [20,29]. Some research suggests
that iodine supplementation might be helpful for fibrocystic
breast disease .
In a double-blind study, researchers randomly assigned women
with fibrocystic breast disease to receive daily supplements of
iodine or placebo for 6 months . At treatment completion,
65% of the women receiving iodine reported decreased pain
compared with 33% of women in the placebo group. A more recent
randomized, double-blind, placebo-controlled clinical trial had
similar findings . Women receiving supplemental iodine had
a significant decrease in breast pain, tenderness, and nodularity
compared with those receiving placebo. The researchers also
reported a dose-dependent reduction in self-assessed pain. None
of the doses was associated with major adverse events or changes
in thyroid function test results though large doses of iodine were
Iodine deficiency has been proposed to play a causative role
in the development of breast cancer [31,32]. Dietary iodine has
also been previously proposed to play a protective role in breast
cancer . The importance of iodine in breast cancer is further
emphasized by the adjuvant effects of iodine  supplementation
in combination with doxorubicin for breast cancer treatment .
In these studies, iodine treatment resulted in reduced tumor size
and proliferating cell nuclear antigen [PCNA] expression.
Based on the importance of iodine in thyroid and breast
health, fetal brain development, as well as deficits in nutritional
trends among younger women, iodine testing and management
may be considered as a potentially important aspect for clinical
Primary sources of iodine for a non-vegan are seafood and
dairy products. Milk contains iodine due to the teat cleansers
typically used in dairies. Since a plant-based diet does not include
seafood or dairy, concern has been expressed that vegans may be
more prone to iodine deficiency.
Studies show that many vegans have very adequate iodine
status, but some are deficient . The median, however, is mildly
deficient. In a study of vegans in the Boston area, median urinary
iodine concentration of vegans was lower than for omnivores
[78.5 μg/liter] but the range varied enormously [6.8 - 964.7 μg/
liter] . However, it is reassuring that this was not associated
with thyroid dysfunction. Other studies of thyroid function in
vegetarians and vegans are limited. Although serum TSH was
generally normal in 101 British vegans, the geometric mean
was 47% higher than for omnivores . No thyroid function
abnormalities were found in studies Swedish and Finnish vegans
[39,40]. Although thyroid function is usually good, patients on a
plant-based diet should be evaluated for iodine sufficiency if their
diet seems to inadequate or if they eat foods grown in a region
with low iodine content. A change in composition in the diet or
supplements may be called for. Iodized salt can also be a good
source of iodine.
Although urinary iodine concentration is a sensitive indicator
of recent iodine intake, laboratory tests that detect abnormal
thyroid function may be more useful for diagnosing chronic iodine
deficiency or excessive iodine intake, and for monitoring the
effects of iodine supplementation . Thyroid function testing
includes measurement of serum concentrations of the following:
a. Thyroid-stimulating hormone [TSH]
b. Thyroxine [T4] as total T4 and free [ie, unbound] T4
c. Thyroglobulin [Tg]
d. Triiodothyronine [T3] as total T3 and free T3 [FT3]
Results from thyroid function studies are usually within
the reference range in the presence of mild iodine insufficiency.
However, in patients with euthyroidism and iodine deficiency,
serum TSH levels may be normal to increased, T3 levels may
be normal or slightly elevated, and T4 levels may be normal
or decreased. Only in very extreme iodine deficiency does
hypothyroidism develop, accompanied by an elevated serum TSH
value and decreased T3 and T4 levels .
Cruciferous vegetables such as broccoli, cauliflower, and
cabbage naturally release a compound called goitrin when
they’re hydrolyzed or broken down. Goitrin can interfere with the
synthesis of thyroid hormones. However, this is usually a concern
only when coupled with an iodine deficiency . Importantly,
heating cruciferous vegetables denatures much or all this potential
goitrogenic effect  (Tables 2 & 3).
Soy is another potential goitrogen. However, numerous studies
have found that consuming soy doesn’t cause hypothyroidism
in people with adequate iodine stores [45,46,47]. Randomized
controlled intervention trials in iodine- and iron-deficient
populations have shown that providing iron along with iodine
results in greater improvements in thyroid function and volume
than providing iodine alone .
* Adequate Intake (AI)
Seaweeds can be very high in iodine. It is possible some
seaweed dishes may exceed the tolerable upper iodine intake
level of 1100 microg/d . One study showed the iodine content
surveyed for nori was 29.3–45.8 mg/kg, for wakame 93.9–185.1
mg/kg, and for kombu 241–4921 mg/kg. Kombu has the highest
average iodine content 2523.5 mg/kg, followed by wakame [139.7
mg/kg] and nori [36.9 mg/kg] . Patients who eat a lot of
seaweed should counseled to consume them in moderation.
Many multivitamin/mineral supplements contain iodine in the
forms of potassium iodide or sodium iodide. Dietary supplements
of iodine or iodine-containing kelp [a seaweed] are also available.
One study found that potassium iodide is almost completely
[96.4%] absorbed in humans . Currently, it is estimated that
only 51% of the types of prenatal multivitamins marketed in the
United States contain iodine  and according to 2001–2006 NHANES data, 15% of lactating women and 20% of non-pregnant
and pregnant women in the United States take a supplement
containing iodine . Several medications can have drug
interactions with iodine supplements. Angiotensin-converting
enzyme [ACE] inhibitors, such as benazepril [Lotensin®],
lisinopril [Prinivil® and Zestril®], and fosinopril [Monopril®.
Taking potassium iodide with ACE inhibitors can increase the
risk of hyperkalemia [elevated blood levels of potassium] .
Taking potassium iodide with potassium-sparing diuretics, such
as spironolactone [Aldactone®] and amiloride [Midamor®], can
increase the risk of hyperkalemia .
Millions of people in America suffer from hyperthyroidism
and hypothyroidism, most commonly in the form of Grave’s
disease or Hashimoto’s Thyroiditis. While in some cases this may
be due to lack of iodine, in the United States it is more likely due
to obesity or an auto-immune disease. The risk of both is reduced
with a plant-based diet. Reducing the risk of hypothyroidism and
hyperthyroidism with a plant-based diet would seem especially
advantageous since a plant-based diet is safe, has no adverse
reactions or contraindications, and can treat and prevent several
common comorbidities such as rheumatoid arthritis, Crohn’s
disease, type II diabetes and coronary artery disease. The
preventative benefit of a plant-based diet might be stronger if
the typical iodine intake were a bit higher. However, while those
following a plant-based diet can be mildly deficient in iodine,
the compensatory mechanism of increased production of TSH
reduces the risk for vegans to experience thyroid dysfunction due
to any lack of iodine.