Subclinical hypothyroidism (SCH) is defined as a normal serum FT4 and a high serum thyrotro-pin (TSH) concentration. Up to 30% of patients with SCH may have vague, non-specific symp-toms of hypothyroidism, but attempts to identify the patients on the basis of clinical finding have not been successful, so the diagnosis can only be made with laboratory testing. The causes of SCH are the same as those of overt hypothyroidism. Most patients have Hashimoto’s thyroiditis. The worldwide preva-lence of SCH ranges from 1-10%. A substantial proportion of patients with SCH develop overt hypothyroidism. Serum TSH concentration and positive antithyroid antibodies (ATA) are sig-nificant predictors of progression to clinical hy-pothyroidism. Some patients with SCH have some symptoms of hypothyroidism, while some studies show significant improvement in hypo-thyroid symptom scores and psychometric test-ing; others found no improvement in symptoms with levothyroxine therapy. There is consensus that SCH in pregnancy is a risk factor for poor developmental outcomes in the offspring and the condition should be treated in women who wish to become pregnant. There is also agreement that patients with SCH and TSH levels over 10 mU/L, or with goiter should be treated. Population-based screening for SCH is not warranted, but thyroid function should be tested in high risk groups, e.g. in women aged over 60 yr, persons with previous radiation therapy of thyroid gland or external radiation, those with previous thyroid surgery of thyroid dysfunction, type 1 diabetes mellitus pa-tients and those with a family history of auto-immune disease. Evidence documenting routine determination of TSH in pregnant women or women planning to become pregnant are insuf-ficient, and it would be reasonable to consider TSH measurement in those at high risk for thy-roid dysfunction.
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