A Healthy Diet, a Healthy Thyroid

Hypothyroidism is a relatively common endocrine disorder, often encountered in the outpatient setting. It may be either subclinical or overt. An elevated TSH above 10mIU/L with a below normal free thyroxine (T₄) designates overt hypothyroidism, whereas subclinical hypothyroidism is defined by a subnormal TSH in conjunction with a normal free T₄ level. According to the National Health and Nutrition Examination Survey (NHANES III), between1998-2004 the prevalence of subclinical disease in a randomly selected U.S. population above 12 years of age was 4.3%. The prevalence of overt hypothyroidism was 0.3%. The Colorado thyroid disease prevalence survey reported the prevalence of subclinical hypothyroidism to be as high as 8.5%. The prevalence of clinical hypothyroidism was found to be 0.4%. Lastly, Framingham data indicates that 5.9% of women and 2.3% of men over 60 years of age have had TSH levels above 10mIU/L [1].

Screening guidelines vary. The American Thyroid Association (ATA) recommends that all adults be screened for hypothyroidism every 5 years, starting at the age of 35. The American Association of Clinical Endocrinologists (AACE) recommends TSH measurement in the elderly, and the American Academy of Family Physicians (AAFP) advocates screening in asymptomatic patients above the age of 60. On the other hand, the U.S. Preventive Services Task Force (USPSTF) advises against adult screening [1]. Nonetheless, the number of TSH assays ordered annually is bound to increase, thus indentifying more and more individuals with TSH abnormalities. With an ever growing amount of information available at one’s fingertips, patients not only look to their physicians for guidance with regards to their medical management, but also for guidance with regards to various environmental/nutritional factors that may affect their thyroid function.

Iodine, a trace element found in rocks, seawater and certain soil, is crucial for the production of thyroid hormone. Data from as far back as the early 1900s shows that thyroid goiters can be prevented by iodine supplementation. Indeed, public health programs such as universal salt iodination have been successful in doing so. The iodine content of most commonly consumed foods is quite low (estimated to be 3-80μg of iodine per serving). Marine animals and plants absorb iodine from sea water, and consequently large consumption of certain types of seaweed can provide an iodine intake of 50-80mg per day [2]. In one study, Teas et al analyzed 12 different species of sea vegetables. They found the iodine content to range from 16μg/g in nori (Porphyra tenera), to greater than 8000μg/g in processed kelp granules [3]. The amount of iodine in diary products is influenced by the use of iodine-containing compounds in fertilizers, livestock feeds and irrigation. Using iodophors to clean teats and milk cans can also increase iodine content. Boiling and baking iodated salt containing foods seems to cause only a small decrease in their iodine content (<10%). Caution must be advised with regards to over supplementation. The ingestion of many grams of iodine can lead to acute iodine poisoning, which can manifest with vomiting, diarrhea, abdominal pain, gastrointestinal irritation and even induce a coma. Excessive intake can also result in a skin disorder called iodermia, which involves the development of a puritic rash, urticaria and acneiform eruptions. In patients with an underlying thyroid disease, high supplementation doses in the microgram range may eventually lead to either hyper- or hypothyroidism [2]. The recommended daily allowance for iodine is outlined in Table 1.

  

    
Age 
    
Male 
    
Female 
    
Pregnancy 
    
Lactation 
  
  

    
Birth    to 6 months 
    
110 mcg*
    
110 mcg*
    
 
    
 
  
  

    
7–12    months
    
130 mcg*
    
130 mcg*
    
 
    
 
  
  

    
1–3    years
    
90 mcg
    
90 mcg
    
 
    
 
  
  

    
4–8    years
    
90 mcg
    
90 mcg
    
 
    
 
  
  

    
9–13    years
    
120 mcg
    
120 mcg
    
 
    
 
  
  

    
14–18    years
    
150 mcg
    
150 mcg
    
220 mcg
    
290 mcg
  
  

    
19+    years
    
150 mcg
    
150 mcg
    
220 mcg
    
290 mcg
  
  

    
*Formula and food should be the only sources of    iodine for infants
  

Table 1:  Recommended Dietary Allowances (RDAs) for Iodine [4].