During treatment of primary hypothyroidism, the respective values of hormonal assays (total and free T4 and T3, ultrasensitive TSH assay, TRH test) and of the serum markers from peripheral tissue response to thyroid hormones used to detect overdosage are matter to debate. Thirty-four female patients with hypothyroidism, aged 38 +/- 7 years (mean +/- SD) were studied. the mean dose of L-thyroxine required to obtain clinical euthyroidism was 100 +/- 35 micrograms/day. Thyroid hormone and TSH concentrations were measured 20 to 24 hours after the last dose of L-thyroxine and after fasting overnight and were compared with the concentrations obtained in 45 euthyroid controls aged 32 +/- 10 years.