If you're suffering from hypothyroidism, there is no reason to avoid vegetables.
Diet quackery has existed for many years and as interest in weight and fitness issues increases, there is a rise in the number of fads. I often hear people avoiding certain foods and vegetables, such as soy, turnips, cabbage, cauliflower, broccoli and brussel sprouts due to an underlying fear that these foods interfere with the synthesis of the thyroid hormone which is responsible for metabolism. The belief is that this promotes weight gain. Those diagnosed with an established under-active thyroid (hypothyroidism) in particular seem to be following this practice although it is not limited to them.
The origin of this fad may be that soy, cabbage, turnip, cauliflower, radish, broccoli, brussel sprouts, rutabaga, kale and kohlrabi contain substances known as goitrogens. Goitrogens are substances that suppress the function of the thyroid gland by interfering with the iodine uptake, which can cause an enlargement of the thyroid, known as goitre. In other words, goitrogens inhibit the thyroid hormone synthesis. But the effect of goitrogens is not significant as most are destroyed whilst cooking. Also, their effect is only relevant if goitrogenic foods are taken in large amounts as staple foods.
However, goitrogens can be a a potential problem if a person's diet is iodine-deficient. A dietary excess of calcium and fluorine is also known to enhance the effect of goitrogens.
Simply put, though goitrogens lower iodine uptake, they are not an important cause of goitre. But if an individual's iodide intake is insufficient (or if there is a large amount of goitrogens in his diet), the thyroid gland slows down the production of its hormones, lowers metabolism and eventually enlarges in an attempt to take up more iodide from the bloodstream.
Several studies done on goitrogenic foods have demonstrated insignificant effects on metabolism. However, a recent study (2010) reported that catechins (flavonoids) in green tea extracts are goitrogenic and had an antithyroid effect, when consumed in high doses. Another study (2004) reported that fermentation of millets may cause goitre due to removal of considerable amounts of certain essential minerals including magnesium, zinc and potassium, and chemical transformation of goitrogens in millets.
However, most vegetables and foods in the goitrogenic list can be consumed freely by all including those who have established thyroid abnormality. They are reservoirs of valuable nutrients and antioxidants; and avoiding them could lead to missing out on these.