Background: Sub clinical hyperthyroidism is a state of subnormal serum TSH and T3,T4 within normal range, although usually without overt clinical manifestation but many disastrous complications especially in senile patient. In Iranian people, serum TSH is generally assayed by IRMA method. This study is aimed to determine the value of low serum TSH in these patients, better management and decision when encountered.

Materials and Methods: The populations under study are guys with serum TSH lower than 0.5mu/l and normal thyroid hormones without known thyroidal and non-thyroidal illness. A basal serum TSH and TSH 30 minutes after TRH injection intra venous were sampled and correlation of clinical signs and symptoms and basal TSH with sub clinical hyperthyroidism was considered.

Results: The population under study was categorized into five groups and prevalence of sub clinical hyperthyroidism was noted. In patients with b.TSH equal or lower than 0.1mu/l, 100%, 0.1-0.2mu/l, 75%, 0.2-0.3mu/l, 38.5%, 0.3-0.4mu/l, 14.3% and TSH levels greater than 0.4mu/l, were all normal. After analyzing of these data and determination of sensitivity and specificity of IRMA, it was concluded that IRMA is not sufficient to distinguish sub clinical hyperthyroidism, although there is a good linear (r=0.68; P<0.001) and cubic (r=0.79; P<0.001) relationship between b.TSH and d.TSH.

Conclusion: Since TRH test is not cost effective for all cases, TSH levels lower than 0.25mu/l, can be considered as sub clinical hyperthyroidism and levels more than 0.4mu/l, as normal. In cases with TSH level between 0.25 and 0.4mu/l, TRH test is needed in high-risk patients.