Hypothyroidism impairs colonic motility and function. An experimental study in the rat.

Hypothyroidism (HT) may be associated with either megacolon or severe constipation that mimics Hirschsprung's disease. The exact nature of this relationship is unclear. This report evaluates colonic motility in experimental HT. HT was produced by a total thyroidectomy in 20 male Sprague-Dawley rats (200-300 g). Ten of the HT rats were given thyroxin (30 micrograms/kg/day i.p. for 4 weeks) for a replacement study (RS). Ten sham operated rats (SH) and 50 unoperated rats (UN) were studied as controls. HT was documented at 4 weeks following total thyroidectomy by measuring serum T3, T4 uptake, and TSH. At four to 6 weeks the animals were evaluated for several studies concerning colonic motility. Normal weight gain was significantly impaired under HT. In addition, daily stool volume as well as the number of fecal pellets were significantly reduced in HT. A barium enema showed a dilated colon in HT with an increase of recto-colonic ratio. Anal canal pressure was relatively low in HT. HT rats had a decreased frequency of rhythmic colonic activity (cycle/min) (HT: 10.22 +/- 2.16 vs. UN: 18.09 +/- 4.79, SH: 15.88 +/- 3.92, p less than 0.001). RS rats showed a recovery of rhythmic frequency (15.56 +/- 4.46, p less than 0.001). A positive recto-anal reflex was seen in all UN and SH rats, but only 40% in HT and 80% in RS. The effect of isoproterenol on anal canal pressure was significantly less in HT. Colonic transit at 4 hours was considerably slower in HT (HT: 29.0 +/- 21.0% vs. US: 60.4 +/- 18.3%, SH: 58.7 +/- 14.8%, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)