Isotopic Renal Function Studies in Severe Hypothyroidism and after Thyroid Hormone Replacement Therapy

Abstract

Aim: To investigate the possible changes in the renal tubular function in severe short-term hypothyroidism using ^99mTc-MAG₃ renography. Methods: 27 consecutive thyroidectomized patients (7 males and 20 females) aged 19–79 (mean 53) years were included in the present study. ^99mTc-MAG₃ renography was performed in all patients before and after thyroid hormone replacement therapy. In addition, ⁵¹Cr-EDTA clearance and serum creatinine concentrations were determined. Results: The serum creatinine concentrations were significantly increased in hypothyroidism as compared with the concentrations after thyroxine substitution (1.30 ± 0.44 vs. 1.04 ± 0.32 mg/dl, p < 0.05). According to the ⁵¹Cr-EDTA clearance, the glomerular filtration rate was significantly lower in hypothyroidism than after treatment (61 ± 18 vs. 75 ± 23 ml/min). In contrast, we did not find any significant change in the renographic parameters for ^99mTc-MAG₃ before and after treatment (total excreted activity 20 min after administration 51 ± 12 vs. 54 ± 14%; T_max left:right 4.2 ± 1.77 : 3.91 ± 1.06 min vs. 4.1 ± 1.66 : 4.4 ± 1.96 min). Conclusions: We did not find any influence of thyroid hormones on the outcome of ^99mTc-MAG₃ renography. As ^99mTc-MAG₃ reflects the tubular function, it seems that the renal hemodynamic changes in severe hypothyroidism mainly affect the glomerular function. In general, the glomerular filtration rate reduction seems to be reversible after hormone substitution therapy; however, care has to be taken in patients with renal insufficiency.