The secretion rates of T4, T3, and rT3 were studied in experiments of short duration by a new method based on determinations of the hormone difference across the thyroid combined with simultaneous electromagnetic thyroid blood flowmetry during surgery in 70 euthyroid patients.
The secretion rate of T3 was similar in normal thyroid tissue and nodular goitre, but those of T4 and rT3 were lower in nodular goitre and solitary adenoma (P < 0.05). In 61 patients with normal thyroid tissue or nodular goitre the secretion rates during surgery (mean ± sem) were for T4 222 ± 28 nmol/day, for T3 27.4 ± 3.1 nmol/day, and for rT3 3.5 ± 0.5 nmol/day. In relation to the individual T4 secretion rate, the secretion rate of T3 was 12.5 ± 3.0% and that of rT3 1.2 ± 0.9%.
In these short-term experiments we found a secretion rate for T4 during operation about 50% greater than in earlier long-term kinetic studies, but which tallied with a recent report using a 4-compartment model. For T3 and rT3 it was 2–3 times greater than earlier estimates. The secretion was estimated to be 50% of the total production rate for T3 and 6% for rT3. If proportional adjustment were performed to yield a T4 secretion of about 130 nmol/day, T3 and rT3 secretion rates would still be greater than earlier reported.