Renal handling of iodothyronines in acromegaly

Measurement of the free serum concentration, the 24-h urinary excretion and the renal clearance of T4, T3, 3,3',5'-tri-iodothyronine (rT3), 3',5'-diiodothyronine (3',5'-T2) and 3,3'-di-iodothyronine (3,3'-T2) was performed in 13 patients with active acromegaly and in 18 healthy controls. The acromegalic patients had normal serum levels of the free iodothyronines, whereas the urinary excretion of T4 and T3 was increased approximately two-fold in the patients with acromegaly. The creatinine clearance, reflecting the glomerular filtration rate (GFR), was increased in the acromegalic patients, in median 133 ml/min versus 87 ml/min (p less than 0.01) in the controls. Compared to the creatinine clearance the clearance of T3 and 3,3'-T2 was higher (p less than 0.01) in acromegalics as well as in controls. The patients with acromegaly had higher renal clearance of T4 and T3 than controls, in median 81 ml/min versus 33 ml/min, and 269 ml/min versus 137 ml/min, respectively (p less than 0.01). These differences were not due to changes in creatinine clearance. The renal clearance of 3',5'-T2 tended to be enhanced in acromegalic patients (8.2 ml/min versus 3.9 ml/min, p less than 0.10), both before and after correction for creatinine clearance. The data suggest that in acromegaly, as in normal condition, iodothyronines are subject to both glomerular filtration and active tubular transport mechanisms. Further, active acromegaly results not only in increased GFR, but also in changes of the net tubular transport in favour of secretion of at least T4 and T3, and possibly also of 3',5'-T2.