Constipation: Causes, Laxative Types & When to Investigate

Common Causes

Laxative Types

References

Sources cited on this page. PubMed links open the original abstract.

1. Higgins PD, Johanson JF. Epidemiology of constipation in North America: a systematic review. Am J Gastroenterol. 2004;99(4):750–759. PMID 15089911 · doi:10.1111/j.1572-0241.2004.04114.x

When constipation signals something serious

Constipation is extremely common and usually reflects diet, dehydration, or lifestyle factors. However, certain patterns should trigger urgent investigation:

• New constipation after age 50 – particularly with any rectal bleeding, mucus in stool, or unintentional weight loss. This is the classic presentation of colorectal cancer until proven otherwise. NICE guidelines recommend a 2-week-wait colonoscopy referral in these circumstances.
• Alternating constipation and diarrhoea – can indicate irritable bowel syndrome (IBS) but also inflammatory bowel disease (IBD) or colon cancer. Faecal immunochemical test (FIT) detects blood not visible to the naked eye and is a valuable triage tool.
• Sudden onset after years of normal bowel habit – any change in bowel habit lasting more than 3 weeks that cannot be explained by a clear trigger warrants investigation.
• Constipation with severe bloating and vomiting – can signal a bowel obstruction, a surgical emergency.

Blood tests used to investigate constipation

For chronic constipation, the following blood tests help identify treatable medical causes:

• Thyroid function (TSH): Hypothyroidism (underactive thyroid) slows gut motility causing constipation, weight gain, fatigue, and cold intolerance. It is one of the most commonly missed reversible causes – and is fully treatable with levothyroxine.
• Calcium: Hypercalcaemia (raised blood calcium) slows bowel motility. Causes include primary hyperparathyroidism, vitamin D toxicity, sarcoidosis, and malignancy. A corrected calcium above 2.6 mmol/L warrants further investigation.
• FBC and CRP: Anaemia with constipation raises suspicion for colorectal cancer. A raised CRP alongside constipation suggests IBD or another inflammatory process.
• Coeliac screen (anti-TTG antibodies + total IgA): Coeliac disease more typically causes diarrhoea but can present with constipation, particularly in adults. It is significantly under-diagnosed.
• Faecal immunochemical test (FIT): A stool test, not a blood test – detects trace amounts of blood in stool. A positive result triggers urgent colonoscopy. It is the NHS's primary screening tool for bowel cancer.

Rome IV criteria and the constipation subtypes

Clinicians classify functional constipation using the Rome IV criteria – at least two of the following for at least 3 months: straining in more than 25% of defecations, lumpy or hard stools, sensation of incomplete evacuation, sensation of blockage, use of manual manoeuvres (digitation), or fewer than 3 defecations per week. Functional constipation is then subtyped by anorectal manometry and transit studies into normal transit, slow transit, or dyssynergic defecation (pelvic floor dysfunction) – because these subtypes respond to different treatments.