Nucleic acids, both DNA and mRNA, have been detected in the circulation and have been demonstrated to be useful in such areas as fetal medicine, oncology, and transplantation. When mRNA is measured in circulating blood, the results are expressed in relation to a reference gene product in order to correct for any differences in extraction, volume of starting material or other differences. Many authors use beta-actin mRNA and express results as a ratio of target mRNA to beta-actin mRNA. We have used a similar approach when studying diabetic retinopathy. Recently, we planned to investigate the expression of thyroid dependent gene expression in acutely ill patients. As a control study, we examined the expression of thyroid hormone-dependent gene expression in subjects with hyperthyroidism and found that the expression of beta-actin mRNA was affected by thyroid hormone status. Blood samples were taken into PAX genetrade mark tubes from 31 healthy subjects (mean age, 43 +/- 16 yrs) and 7 patients with hyperthyroidism (mean age, 43 +/- 5 yrs). Diagnosis of hyperthyroidism was confirmed by clinical findings and biochemical results. After extraction of mRNA, cDNA was synthesized using reverse transcription. Quantification of Na/K-ATPase, T3 receptor, and beta-actin cDNA was carried out by RT-PCR. Median beta-actin levels were significantly higher in hyperthyroid subjects compared to healthy subjects (18.2 versus 2.30; P < 0.00042). When mRNA for the T3 receptor was expressed in relation to beta-actin, there was a significantly higher in hyperthyroidism (0.0168 versus 0.218, P < 0.05). However, this was significantly lower when expressed in relation to total RNA (12.2 versus 2.24, P < 0.00015). We conclude that normalizing results to beta-actin may not be appropriate in all circumstances.