BACKGROUND: Chronic constipation, diarrhea, and fecal incontinence (FI) are prevalent with significant impact on quality of life and healthcare utilization. Thyroid dysfunction was recognized as a potential contributor to bowel disturbances in selected populations, but the strength/consistency of this association remain unclear.

AIMS: To investigate the relationship between thyroid function and bowel health measures (constipation, diarrhea, and FI) in a nationally representative sample of the U.S.

METHODS: We conducted a cross-sectional analysis of the National Health and Nutrition Examination Survey (NHANES) in 2005-2008. Adults aged ≥ 20 with TSH, free T4 (fT4), and bowel health data were included. Multivariable logistic regression models were constructed to examine associations between thyroid function and bowel health measures, adjusting for potential confounders.

RESULTS: Among 6,552 participants, 93.6%, 4.3%, 1.7%, and 0.4% had TSH levels of 0.24-5.4, 5.4-10, < 0.24, and ≥ 10 mIU/L, respectively. There were no significant differences in prevalence of thyroid dysfunction between individuals with constipation or diarrhea and those without. In fully adjusted models, TSH was not a significant predictor of constipation, diarrhea, or FI, but higher fT4 was protective against constipation as a continuous variable (OR 0.47, CI: 0.26-0.85, p = 0.014)). Neither hypothyroid nor hyperthyroid status significantly correlated with constipation or diarrhea, although hyperthyroidism was associated with FI (OR 3.58, CI:1.51-8.49, p = 0.005).

CONCLUSION: While bowel disturbances were common in this nationally representative sample, the yield of thyroid function testing was low. Overt hypo- and hyperthyroidism were not significantly associated with constipation or diarrhea. Clinical utility of routine thyroid testing for bowel symptoms may be low in patients without systemic manifestations of severe thyroid disease.