Subclinical thyroid disease is defined as elevated or depressed serum thyrotropin (TSH) level with a normal serum-free thyroxine (T4) level [1].
PREVALENCE OF SUBCLINICAL HYPOTHYROIDISM
The prevalence of subclinical hypothyroidism varies with population, age, sex, race, region, and method of TSH measurement. TSH is heterogeneous with respect to both glycosylation and biological activity [2].
On the other hand, one German study that included only healthy persons with normal thyroid function and morphology based on sonography [37] proposed a normal TSH range of 0.3 to 3.7 mIU/L, which is quite lower than the range reported in the United States. The difference may be related to mild iodine-deficient status in German populations [37].
It is known that subclinical hypothyroidism is more prevalent in iodine-sufficient areas [26,27,28]. In a study with elderly subjects in Denmark, 3.8% of subjects in the area of low iodine intake had high serum TSH, while 18% in the area of high iodine intake showed subclinical hypothyroidism [26].
PREVALENCE AND RISK FACTORS OF SUBCLINICAL HYPERTHYROIDISM
The major difference in risk factors between hyperthyroidism and hypothyroidism is the amount of iodine intake; hyperthyroidism is more common in iodine-insufficient areas [26,27]. Thyroid autonomy is inversely correlated with the iodine intake [60].
CONCLUSIONS
In summary, subclinical thyroid disease is more prevalent in the elderly and in women. The iodine intake is inversely correlated with thyroid function; deficient iodine intake is related with hyperthyroid status, while excessive or sufficient intake of iodine is associated with hypothyroidism. Thyroid autoantibodies, smoking status, environmental temperature and ethnicity are also risk factors for subclinical hypothyroidism.
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