Can Atapex and Synthroid Be Taken Together?
Yes, a patient can take Atapex and Synthroid (levothyroxine) together, but they must be separated by at least 4-6 hours to prevent potential binding interactions that could reduce levothyroxine absorption. If Atapex is a bile acid sequestrant therapy (BAST), this timing separation is critical for maintaining adequate thyroid hormone levels.
Critical Timing Requirements
- Administer levothyroxine at least 1 hour before or 4-6 hours after bile acid sequestrants to minimize the risk of reduced absorption, as recommended by Health Canada 1
- Gastric emptying studies suggest a 3-hour window between administration is adequate to avoid binding interactions 1
- Take levothyroxine on an empty stomach, typically in the morning, and delay Atapex until later in the day to maximize separation and ensure consistent levothyroxine absorption 1
Monitoring Requirements for Concurrent Use
- Monitor serum TSH levels more frequently when initiating or adjusting bile acid sequestrant therapy, as levothyroxine requirements may increase due to reduced absorption 1
- Recheck TSH and free T4 in 6-8 weeks after starting the bile acid sequestrant to assess if levothyroxine dose adjustment is needed 2
- Patients may require increased doses of levothyroxine when taking bile acid sequestrants concurrently, as these agents can bind thyroid hormone in the gastrointestinal tract 1
Specific Drug Interaction Considerations
- Cholestyramine and colestipol have significant potential to bind levothyroxine, requiring strict dose separation 1
- Colesevelam has reduced interaction potential compared to older bile acid sequestrants but still requires dose separation for levothyroxine 1
- Colesevelam has been found to reduce absorption of levothyroxine specifically, necessitating the 4-6 hour separation 1
Clinical Pitfalls to Avoid
- Never assume adequate levothyroxine absorption without monitoring TSH when bile acid sequestrants are added to therapy, as 25% of patients on levothyroxine are unintentionally maintained on inadequate doses 2
- Failing to separate administration times can lead to subclinical or overt hypothyroidism despite appropriate levothyroxine dosing 1
- Do not adjust levothyroxine dose based on symptoms alone—confirm inadequate replacement with TSH and free T4 measurements before increasing the dose 2
- If TSH becomes elevated (>4.5 mIU/L) after starting the bile acid sequestrant, increase levothyroxine by 12.5-25 mcg and recheck in 6-8 weeks 2
Target TSH Range During Concurrent Therapy
- Maintain TSH within the reference range of 0.5-4.5 mIU/L for patients taking levothyroxine for hypothyroidism 2
- Both undertreatment (TSH >10 mIU/L) and overtreatment (TSH <0.1 mIU/L) carry significant morbidity risks, including cardiovascular complications and bone loss 2, 3
Alternative Considerations
- If adequate TSH control cannot be achieved despite dose separation and levothyroxine adjustment, consider switching to a different bile acid sequestrant with lower binding potential 1
- For patients with difficulty adhering to complex medication schedules, simplify the regimen by taking levothyroxine immediately upon waking and the bile acid sequestrant with meals 1