I have been on low‑dose methimazole for 18 years for a goiter and was biochemically euthyroid with mildly suppressed TSH; I stopped methimazole before a scheduled colonoscopy—what should I do regarding the procedure and thyroid management?

Management After Methimazole Discontinuation Before Colonoscopy

Proceed with Colonoscopy as Scheduled

You should proceed with your colonoscopy as planned, since methimazole discontinuation before the procedure was appropriate and does not require rescheduling. The 2025 US Multi-Society Task Force guidelines make no specific contraindications regarding thyroid status for colonoscopy, and your biochemically euthyroid state with mildly suppressed TSH does not pose procedural risks 1.

Colonoscopy Preparation Recommendations

• Follow standard bowel preparation with a split-dose regimen, consuming the second portion 4–6 hours before colonoscopy and completing at least 2 hours before the procedure 1.
• Use dietary modifications limited to the day before colonoscopy only, including low-residue and low-fiber foods or full liquids for early and midday meals 1.
• No medication adjustments beyond what you've already done are necessary for the colonoscopy itself 1.

Thyroid Management Strategy After Colonoscopy

Immediate Post-Procedure Assessment (Within 3–6 Weeks)

Recheck TSH and free T4 levels 3–6 weeks after stopping methimazole to determine if thyroid function has changed, since 30–60% of mildly abnormal thyroid values normalize spontaneously and your 18-year treatment may have allowed thyroid recovery 2.

• Measure both TSH and free T4 to distinguish between continued euthyroid status, subclinical hypothyroidism (elevated TSH with normal free T4), or overt hypothyroidism (elevated TSH with low free T4) 2.
• If TSH remains in the normal range (0.45–4.5 mIU/L) with normal free T4, no treatment is needed and you can continue monitoring 2.

Decision Algorithm Based on Post-Discontinuation Results

If TSH >10 mIU/L with normal or low free T4:

• Restart levothyroxine therapy immediately at 1.6 mcg/kg/day (if under 70 years without cardiac disease) or 25–50 mcg/day (if over 70 years or with cardiac disease), as this TSH level carries approximately 5% annual risk of progression to overt hypothyroidism 2.
• Recheck TSH and free T4 in 6–8 weeks after starting levothyroxine 2.

If TSH 4.5–10 mIU/L with normal free T4:

• Monitor without treatment if asymptomatic, rechecking TSH every 6–12 months 2.
• Consider levothyroxine trial if you develop fatigue, weight gain, cold intolerance, or other hypothyroid symptoms 2.
• Measure anti-TPO antibodies to assess progression risk—positive antibodies indicate 4.3% annual progression versus 2.6% in antibody-negative patients 2.

If TSH remains normal (0.45–4.5 mIU/L) with normal free T4:

• Continue observation without medication 2.
• Recheck thyroid function annually or if symptoms develop 2.
• Your goiter may have been the primary indication for methimazole rather than hyperthyroidism, and prolonged treatment may have allowed thyroid function to normalize 3, 4.

Special Consideration for Your Goiter

• The goiter that prompted your original methimazole treatment may have been methimazole-induced or pre-existing 3.
• If TSH rises significantly after methimazole discontinuation, this could stimulate goiter growth, requiring monitoring for compressive symptoms like dysphagia or dyspnea 3.
• Methimazole-induced goiter typically improves within 12 months of discontinuation as TSH normalizes 3.

Critical Pitfalls to Avoid

• Do not restart methimazole without confirming current thyroid status—your mildly suppressed TSH on low-dose methimazole for 18 years suggests you may have been overtreated, and stopping may allow normalization 2, 3.
• Do not treat based on a single post-discontinuation TSH measurement—wait 3–6 weeks and confirm with repeat testing, as transient elevations are common during the adjustment period 2.
• Never restart thyroid medication before ruling out adrenal insufficiency if you develop symptoms suggesting central hypothyroidism (though unlikely in your case given your history) 2.
• Avoid assuming you need lifelong treatment—approximately 37% of patients with subclinical hypothyroidism spontaneously normalize, and your 18-year treatment course may have allowed thyroid recovery 2, 4.

Monitoring Timeline

1. Now: Proceed with colonoscopy as scheduled 1.
2. 3–6 weeks post-methimazole: Check TSH and free T4 2.
3. Based on results: Initiate treatment if TSH >10 mIU/L, monitor if TSH 4.5–10 mIU/L, or continue observation if TSH normal 2.
4. 6–8 weeks after any treatment initiation: Recheck TSH and free T4 2.
5. Long-term: Annual TSH monitoring once stable 2.