The significance of subnormal, detectable TSH values: laboratory and clinical aspects

In order to establish the significance of subnormal, detectable TSH values in the range 0.05-0.3 mU/L by sensitive immunoradiometric assay, we assessed 3150 consecutive tests of thyroid function in which TSH was measured on 1400. Sixty TSH values (4.3%) fell in this range and in 80% of these the result was confirmed on repeat assay. Conditions associated with subnormal detectable TSH values were treated hyperthyroidism (21), nonthyroidal illness (17), euthyroid multinodular goitre (10), T4 therapy for primary hypothyroidism (7), and pituitary disease (5). At follow-up 2-15 months later, a second sample showed that TSH remained in this range in only 9 of 41 patients (22%). These findings show a frequency of TSH values intermediate between normality and the suppressed values of hyperthyroidism sufficient to compromise the value of sensitive TSH measurement as the single initial test of thyroid function. While TSH values in the subnormal detectable range rule out hyperthyroidism, such results may merit follow-up in goitrous patients, in whom such a finding can precede overt hyperthyroidism.