Hypothyroidism: Nutritional Considerations
Genetic factors apparently account for approximately 80% of the risk for autoimmune thyroid disease, leaving a role for environmental factors such as diet in many cases. Individuals and populations ingesting inadequate amounts of iodine appear to be particularly at risk, as do patients with celiac disease.
The use of iodized salt is a well–accepted public health strategy for decreasing the incidence of iodine deficiency disorders.1 Although mild iodine deficiency in a population results in enlarged thyroid glands, evidence of clinical hypothyroidism does not necessarily follow.2 Conversely,even mildly to moderately excessive iodine intake (>220 μg/day) through foods, dietary supplements, topical medications, and/or iodinated contrast media3 can increase risk for hypothyroidism.4 Iodine excess causes a hypothyroid state partly because of a decrease in the sodium/iodide symporter5 that is responsible for transport of iodide into thyrocytes, a fundamental step in thyroid hormone biosynthesis.6
Hypothyroidism is not uncommonly found in patients with celiac disease (CD);7 for example, 8% of patients with CD were found to be hypothyroid in one study.8 The prevalence of autoantibodies to the thyroid is significantly higher in patients with undiagnosed celiac disease than in celiac patients on a gluten–free diet.9,10 These autoantibodies appear to be gluten–dependent, disappearing after adoption of a gluten–free diet.9 Although evidence is limited, clinical trials found that most patients who strictly followed a gluten–free diet for one year experienced a normalization of subclinical hypothyroidism11 and reduced need for thyroxine.12
Iron deficiency may also contribute to thyroid disease risk. Although Western, meat–eating populations have greater iron stores than non–Western populations, some individuals may experience poor iron status. Plasma thyroxine and triiodothyronine concentrations were significantly lower in women with iron–deficiency anemia, compared with controls.13 Iron–deficiency anemia blunts the effect of iodine supplementation on thyroid function, and iron supplementation improves it.14 However, iron supplements should be taken apart from levothyroxine (see below).
When thyroid medication is used, it should be taken on an empty stomach. Meals can delay gut absorption of levothyroxine, with a particularly noticeable effect from high–fiber meals.15,16 Both calcium carbonate and iron supplements can significantly reduce absorption of levothyroxine and reduce its effectiveness.17,18 This may have particular relevance for older women, who are more likely to need thyroid hormone replacement and to take calcium supplements.
Dietitian should instruct patient on ways to avoid diet–medication interactions that may influence TSH and T3.
What to Tell the Family
Hypothyroidism is common and treatable, in most cases with excellent outcome. Prevention of hypothyroidism requires adequate dietary intake of iodine and selenium at recommended levels. Patients who live in countries where iodine and selenium are scarce may need supplements of these minerals. In patients with established hypothyroidism, hormone replacement is needed to normalize T3 and TSH levels.
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