If your doctor has ordered thyroid function tests, it’s important to avoid certain foods, medications, and supplements beforehand. Proper preparation can help ensure accurate test results.
Why thyroid tests are done
Thyroid tests check how well your thyroid gland is working. The thyroid is a small, butterfly-shaped gland in your neck that releases hormones to regulate your metabolism. Thyroid disorders are common, especially in women and older adults. Symptoms include fatigue, weight changes, feeling cold, and more.
Your doctor may order thyroid tests if you have symptoms of hyperthyroidism or hypothyroidism. Hyperthyroidism is when your thyroid is overactive and produces too much hormone. Hypothyroidism is when your thyroid is underactive and doesn’t produce enough hormone. Thyroid tests help diagnose these conditions.
Common thyroid tests include:
- TSH (thyroid-stimulating hormone) test – measures TSH levels released by your pituitary gland
- T4 (thyroxine) test – measures thyroxine, the main hormone released by your thyroid
- T3 (triiodothyronine) test – measures T3, another thyroid hormone
- Thyroid autoantibodies – measures antibodies that attack the thyroid
Foods and drinks to avoid before thyroid testing
Certain foods and drinks can interfere with thyroid test results. Here are some things to avoid in the days leading up to your test:
- Iodine – High iodine intake can temporarily lower TSH and thyroid hormone levels. Avoid iodized salt, seafood, seaweed, multivitamins containing iodine, and foods cooked with iodized salt. Stop eating high-iodine foods 2 weeks before testing.
- Biotin – Biotin is a B vitamin in supplements and fortified foods. It can falsely lower TSH and thyroid hormone levels. Avoid biotin supplements and biotin-enriched foods for 2-3 days before testing.
- Soy – Soy can impact thyroid hormone levels in people with hypothyroidism. Limit soybeans, edamame, tofu, and soy milk for at least 3 days.
- Cruciferous vegetables – Vegetables like broccoli, cabbage, and Brussels sprouts contain goitrogens that may interfere with thyroid function. Cook these vegetables thoroughly before eating them.
- Coffee – Caffeine has been found to raise thyroid hormone levels. Avoid coffee and other caffeinated drinks and foods for 1-2 days before testing.
Medications that affect thyroid tests
Many common medications can impact thyroid function and test results. Consult with your doctor about stopping these medications prior to thyroid testing:
| Medication | Effects on Thyroid Tests | Recommendation |
|---|---|---|
| Multivitamins containing iodine or iron | May increase thyroid hormone levels | Stop 2 weeks before testing |
| Amiodarone (Cordarone) | Contains iodine that can change thyroid function | Consult your doctor about stopping |
| Lithium | Can cause hypothyroidism | Consult your doctor about stopping |
| Interferon-alpha | Used to treat hepatitis C and cancer, can disrupt thyroid function | Consult your doctor about stopping |
| Iodine supplements | Increase iodine levels that impact thyroid function | Stop 2 weeks before testing |
| Estrogen | Can increase thyroid binding globulin (TBG) levels | Consult your doctor about stopping |
It’s important not to stop any prescription medication without your doctor’s guidance. Speak to your physician about appropriate washout periods for medications that affect thyroid tests.
Herbal supplements and thyroid testing
Many herbal supplements can interfere with thyroid function and test results. Avoid taking the following supplements for 1-2 weeks before thyroid testing:
- Ashwagandha – May increase thyroid hormone levels
- Bladderwrack – Contains high iodine levels
- Coleus – May increase T3 and T4 levels
- Ginseng – May suppress TSH levels
- Rhodiola – May inhibit thyroid hormone synthesis
When should you stop eating and drinking before a thyroid test?
To get the most accurate results from your thyroid test, follow these recommendations:
- Avoid biotin supplements and biotin-enriched foods for 2-3 days beforehand.
- Stop consuming iodine-rich foods 2 weeks before testing.
- Limit soy, cruciferous vegetables, and caffeine for 1-3 days prior.
- Discontinue herbal supplements 1-2 weeks before your test.
- Speak with your doctor about stopping thyroid-impacting medications.
- Fasting – For some thyroid tests, your doctor may instruct you to fast for 8-12 hours beforehand. Water is okay.
Other factors that influence thyroid test results
In addition to diet and supplements, other factors can affect your thyroid test results:
- Time of day – TSH levels are highest in the morning and lowest in the afternoon/evening. Schedule testing for the same time each time to compare results.
- Stress – High stress can temporarily increase TSH levels.
- Illness – Severe non-thyroidal illness can lower TSH and thyroid hormone levels.
- Pregnancy – Increased estrogen levels during pregnancy raise thyroxine-binding globulin (TBG). Test T4 and T3 uptake for accurate results.
- Medications – Oral contraceptives, corticosteroids, and more can affect thyroid hormone levels.
- Menstrual cycle phase – Thyroid hormone levels fluctuate during the menstrual cycle.
Conclusion
Preparing properly for thyroid testing can help ensure you get accurate results. Avoid iodine-rich foods, soy, supplements, and some medications beforehand. Fasting may be required. Timing, health status, hormones, and other factors can also impact your results. Speak with your doctor about any concerns before your thyroid test.