The effect of thyroxine on small intestinal motility in the elderly

To study the effects of thyroxine on orocaecal transit time in a group of elderly hypothyroid patients on long-term thyroxine replacement therapy.

Measurement of the effect of withdrawal and subsequent replacement of thyroxine replacement therapy on orocaecal transit time.

Fifteen elderly, previously hypothyroid patients on full replacement therapy with oral thyroxine were studied. There were 11 females and four males, aged 60-94 years (median 78 years) receiving initially 50-200 micrograms of oral thyroxine daily (median 100 micrograms).

Serum TSH and FT4 were measured by radioimmunoassay and orocaecal transit time assessed using a lactulose hydrogen breath test. These tests were repeated 7 days after withdrawal of thyroxine replacement and again 7 days after subsequent reinstatement of therapy.

On withdrawal of therapy, the median transit time increased from 75.0 to 135 minutes (P less than 0.01), the median TSH increased from 1.8 to 2.3 mU/l (P = NS) and the median FT4 decreased from 13.7 to 10.6 pmol/l (P less than 0.01). On reinstatement of therapy, the median transit time decreased to 95 minutes (P = NS), the median TSH decreased to 1.1 mU/l (P = NS) and the median FT4 increased to 14.1 pmol/l (P less than 0.01).

These findings demonstrate the sensitivity of the small bowel to changes in thyroid hormone status and suggest that constipation arising as a result of this hypomotility may well be an early physical manifestation of hypothyroidism.