Which are the Highest Normal Levels of TSH?

Some endocrinologists have proposed that the high limit of normality of TSH is to be lowered to 2.5 μU/ml (instead of the traditional value of 45 μU/ml): this suggestion, which has arisen many discussions, derives from the uncertainty about subclinical hypothyroidism prevalence in the general population, on whose values the determination of normality interval is based. In this trial, TSH was determined in 1861 adults in their middle age, casually chosen in the state of Washington, who also underwent anti-thyroid hormones test and thyroid ultrasonography. The 97.5th percentile of TSH (normally considered the cutoff to determine the highest level of normality) was the following:
5.67 μU /ml in the whole cohort.
3.73 μU /ml (1448 patients) after having excluded subjects referring to have had thyroid problems or presenting an abnormal clinical examination.
3.37 μU /ml (1186 patients) after further exclusion of the subjects with anti-thyroid antibodies.
3.37 μU /ml (766 subjects) after further exclusion of subjects with thyroid abnormalities at ultrasonography. To finish, TSH levels were measured with ELISA test (which is now no more used). Performing dosages with a new third-generation system, TSH values were obtained averagely higher of 0.73 μU /ml compared to ELISA method. So, in probably normal subjects, the higher TSH cutoff should be of 4.1 μU /ml (3.37 + 0.73).

We can also accept the conclusions of this trial (4 μU /ml is the higher limit of normality for TSH), but the main aspect remains to be cleared up: from a clinical point of view, is it useful to treat subjects presenting a higher TSH but being completely asymptomatic with thyroid supplementations?