That finding is particularly relevant for women who are pregnant, researchers say, as that is a time when a mom's iodine levels are strained by her growing baby.
"It's an interesting observation that we ought to pay attention to," said Dr. Robert Smallridge, an endocrinologist at the Mayo Clinic in Jacksonville, Florida, who was not involved in the new research.
Though the study was small, the researchers "have identified a group that may be more likely to be iodine insufficient," Smallridge told Reuters Health. "That's an important thing for us to recognize and to counsel our patients about."
Iodine, which is present in iodized salt, seafood, eggs, dairy and some breads, is used by the thyroid gland to help regulate metabolism and development, especially in babies and young kids.
Iodine deficiency during fetal and early-childhood development is a leading cause of brain impairments in much of the world.
Although researchers believe that most people in the U.S. get plenty of iodine in their diets, the American Thyroid Association recommends that pregnant and breastfeeding women take a vitamin with iodine because low iodine can increase the risk of miscarriage and thyroid problems in moms, in addition to mental disabilities in babies.
Dr. Angela Leung of Boston Medical Center, lead author of the new study, said that little research has been done on whether or not vegetarians and vegans may be more likely to have iodine deficiencies because of their dietary restrictions.
As a first stab at that question, she and her colleagues recruited 140 vegetarians and vegans -- mostly women -- and tested their urine for concentrations of iodine.
According to the World Health Organization, the general recommended range of iodine concentrations per liter of urine is between 100 and 199 micrograms, and between 150 and 249 micrograms per liter in pregnant women.
Leung and her colleagues calculated an average iodine level of 147 micrograms in vegetarians and 79 in vegans -- those who avoid not just meat but eggs and dairy products as well.
Researchers also measured the participants' levels of thyroid hormones as a gauge of how well their thyroids were functioning, in addition to levels of a couple of chemicals -- perchlorate and thiocyanate -- known to interfere with iodine in the thyroid.
Thyroid hormone levels were similar in both vegetarians and vegans, and in the normal range, the authors report in The Journal of Clinical Endocrinology and Metabolism. Iodine concentrations were not linked to thyroid function in either group.
Leung said that is probably because the study was very small, which makes it harder for those associations to come out.
There was also no relationship between thyroid hormone levels and urine concentrations of perchlorate, a contaminant in food and water, or of thiocyanate, a chemical found in cabbage-like vegetables and in cigarette smoke.
One limitation of the study is that the urine test for iodine is only a window into recent iodine consumption, and can't get at how long-term iodine levels may be affecting the thyroid.
Leung said the purpose of this study was mainly to make the public aware of the issue of iodine deficiency, especially in women who forego some high-iodine foods, and to open the door for more research into this topic.
"In vulnerable populations, especially women that are pregnant or lactating, we want those populations to be sufficient in iodine," Leung told Reuters Health.
"All women of childbearing age should be encouraged to take iodine supplements... to ensure that the fetus is exposed to additional iodine during development," Leung said. "In particular vegan women of childbearing age should be encouraged to do that."
Too much iodine can also cause thyroid problems. Sarah Bath, a PhD student studying iodine in women at the University of Surrey, U.K., said that people starting supplements should stay away from kelp and seaweed supplements -- which may have widely variable iodine levels. She also said they should have a doctor looking out for them while taking iodine.
Supplements in the form of potassium iodide can be bought for a few cents per capsule.
Smallridge said that this study tells doctors that they should be encouraging their vegan patients to get enough iodine, and possibly trying to identify vegetarian patients who may be at risk of iodine deficiency as well.
"Sometimes iodine gets overlooked with milk-free diets or vegan diets," Bath, who did not participate in the new study, told Reuters Health. "In countries that are iodine sufficient...iodine is not something that people are concerned with because of this overriding assumption that intakes are okay."
Bath cited another recent study, published in The Lancet, which found that half of female adolescents sampled in the U.K. had at least mild iodine deficiency.
From the June 6, 2011, Prepared Foods' Daily News.