A prospective critical evaluation of in vitro thyroid function tests

A number of 2 325 serum samples from a population of in- and outpatients were collected during a six-month period in order to evaluate the usefulness of various thyroid function tests in the clinical laboratory routine. The samples were analysed with the following thyroid function tests: total triiodothyronine (T3) (TT3), total thyroxine (T4) (TT4), free T3 index (FT3I), free T4 index (FT4I) and thyrotropin (TSH). One to two years after the primary evaluation, a follow-up was performed and the final diagnoses were checked in the patients' records. The values of these parameters in the diagnosis of hyperthyroidism were: FT3I greater than FT4I greater than TT3 greater than TT4. The corresponding results in the diagnosis of hypothyroidism were: TSH greater than FT4I greater than FT3I = TT3. No single test could detect both hyper- and hypothyroidism effectively. The only one-step strategy for thyroid evaluation in patients without apparent clinical signs of hyper- or hypothyroidism would therefore be the combined determination of T3 and TSH. The study also showed distinct differences between the reference values of the healthy population and patients without thyroid disorders.