Changes in renal tri-iodothyronine and thyroxine handling during fasting

Objective: Liver handling of thyroid hormones (TH) has been known to alter significantly during fasting. This study investigates whether renal handlin g of TH is also changed during fasting. Methods: We measured urinary excretion rates and clearances of free tri-iod othyronine (T-3) and free thyroxine (T-4) in healthy subjects prior to and on the third day of fasting. Results: During fasting, both mean T-3 and T-4 urinary excretion decreased significantly to a mean value of 42% of control. Also, total and free (F) s erum T-3 concentrations declined significantly, but serum T-4 did not chang e. Both FT3 and FT4 clearance decreased significantly during fasting (62% a nd 42% of control). The fasting-induced decrease in uric acid clearance cor related well with the decrease in FT3 clearance (r = 0.94; P < 0.001). Seru m concentrations of non-esterified fatty acids (NEFA) were significantly el evated during fasting. Conclusions: The findings cannot be fully explained by the fasting-induced decrease in serum T-3 and are in accordance with inhibition of uptake of T- 3 and T-4 at the basolateral membrane of the tubular cell. This inhibition may be caused by a decreased energy state of the tubular cell and by other factors such as ketoacidosis and/or increased NEFA concentrations during fa sting.