Transport of the thyroid hormones across the feline gut wall

Intestinal absorption of radioiothyroxine (T4*) and of radiotriiodothyronine (T3*) was studied in normal cats (n = 5 for T4* and n = 6 for T3*). Absorption was localized by a series of studies comparing the time-activity data for plasma and fecal T4* or T3* radioactivity after i.v. administration with data obtained in the same cats after administration by the oral, jejunal, midileal, and cecal routes. Data were analyzed using a multicompartmental model to account for longitudinal gut transit, metabolic loss, and reversible and irreversible binding, as well as absorption. The relative importance of the gut as a site for T4* or T3* residence at steady state was determined from T4* (n = 6) and T3* (n = 3) content of gut wall and contents at the end of tracer infusion to steady state. Secretion of T4* and T3* from the general pool into the various parts of the gut was calculated from the steady-state data by fitting them to the model using its steady-state parameters. Secretion parameters were adjusted in this fitting process, which established the rate and sites of T4* and T3* secretion into the gut. Overall, feline absorption of T4* and T3* is low compared with absorption in human subjects. Gentamicin treatment enhanced T3* absorption, but general wasting (weight loss associated with time under study) had no effect on either T4* or T3* absorption. The most important site for absorption of both T4* and T3* is the jejunum-upper ileum bowel segment. However, when the mass of gut wall in each segment is taken into account, absorption rate is highest at the duodenum and decreases distally. Secretion of both T4* and T3* is most active into the duodenum, presumably largely through the bile. In addition, there is active secretion of both T4* and T3* into the ileum and of T4* into the colon. These studies indicate that T4* and T3* absorption and secretion balance to make the gut an important locale for both hormones. Because of the cat's relatively low T4 and T3 absorption rates, much of the gut contents are destined for excretion. However, an active secretion-reabsorption loop for both T4* and T3* is present.