Serum-"free"-thyroxine and triiodothyronine levels: their significance in preclinical hyperthyroidism and subclinical hypothyroidism.

A study was performed to establish whether the serum-"free" T4 and T3 levels contributed to the diagnosis of latent thyroid disease. The results suggested that in case of autonomous adenoma an increase in the serum free T3-level is the most reliable indicator of the transformation of the thyroid nodule, and that this hormone concentration, in addition to the TSH--IRMA procedure contributed to the differentiation between euthyroid and hyperthyroid state. In subclinical hypothyroidism, regarded as a heterogeneous subgroup, the mean "free" T4 levels, though still within the normal range, were below those of the controls. Measurement of "free" T4 alone was, however, not sufficient for the diagnosis of subclinical hypothyroidism, but contributed to the differentiation of subclinical from manifest hypothyroidism. Subclinical hypothyroidism raised the possibility that a reduction in the serum hormone levels, though still within the normal limits, may be perceived by the peripheral tissues. Preliminary studies indicated that measurement of the systolic time interval may offer information on this issue.