The thyroid gland keeps your body running smoothly, but it is very vulnerable to the effects of certain everyday drugs and supplements.

Drugs that affect the thyroid: If you take any of these drugs, talk to your doctor about monitoring your thyroid…

Estrogen, inboth hormone replacement therapy and birth control pills, causes changes that require an increase in thyroid-hormone production, which can lead to hypothyroidism. Patients ­already taking levothyroxine (prescription thyroid hormone) may need a dose adjustment after either starting or stopping estrogen. 

Amiodarone for irregular heartbeat can cause too much or too little hormone production as well as abnormal lab results even when thyroid function really is normal. 

Lithium, for bipolar disorder, can cause hypothyroidism

Drugs that affect thyroid tests: 

Biotin, a common B vitamin supplement, can skew levels up or down. Stop taking it several days before blood work.

Blood thinners heparin or enoxaparin or salsalate (an NSAID) can affect thyroid test results in various ways. Talk to your doctor about it, but don’t stop taking them on your own. 

If you take levothyroxine to treat hypothyroidism, wait one hour before eating breakfast and four hours before taking iron- or calcium-containing supplements or foods. If this is impossible with your schedule, take levothyroxine at bedtime.

Even short-term use of proton pump inhibitors (for heartburn) can affect thyroid levels in patients taking levothyroxine. Recheck thyroid labs one to two months after starting or stopping a PPI. Once the labs are normal, they should be monitored every six to 12 months.

Cholesterol drugs cholestyramine and colestipol can reduce levothyroxine absorption. Talk to your endocrinologist about more frequent blood work to monitor your thyroid hormone levels.­