Short answer: It may not be a neutral add-on. The NIH Office of Dietary Supplements says ashwagandha might affect thyroid function and might interact with thyroid hormone medicines. In a trial of 50 people with mild underactive thyroid, 300 mg twice a day for 8 weeks lowered TSH and raised T3 and T4 compared with placebo, and three cases of thyrotoxicosis (too much thyroid hormone) have been reported. Ask your prescriber before combining them.
| Study or report | What happened |
|---|---|
| 3 adult men, 500 mg a day of root and leaf extract for 8 weeks | Small increases in blood T4 |
| 50 people with subclinical hypothyroidism, 300 mg twice a day of root extract for 8 weeks | Lower TSH and higher T3 and T4 than placebo |
| 3 case reports, women aged 32 to 73 | Thyrotoxicosis; one case involved 1,950 mg a day for more than 2 months; all resolved after stopping |
ODS says these findings suggest ashwagandha might interact with thyroid hormone medications. Subclinical hypothyroidism means TSH is high while thyroid hormone levels are still in range.
Levothyroxine doses are set from TSH and, in some cases, free T4. The Synthroid label says the goal is to bring these into range and that the full effect of a dose may take 4 to 6 weeks to show. Anything else that pushes thyroid hormone up can add to the dose. The label lists signs of too much thyroid hormone, including fast or irregular heartbeat, nervousness, trouble sleeping, tremor, heat intolerance and weight loss.
Because of that, starting or stopping ashwagandha between thyroid tests could make a result harder to read. A prescriber may want to know the start date when looking at the next TSH. If you also take biotin, the Synthroid label says to stop it at least 2 days before TSH or T4 tests; see biotin and blood tests.
ODS says ashwagandha might also interact with diabetes, blood pressure, immunosuppressant and sedative medicines. It says ashwagandha appears well tolerated for up to about 3 months, with mild side effects such as stomach upset, loose stools, nausea and drowsiness, but that long-term safety is not known.
It describes reports of liver injury, including five people who took 450 to 1,350 mg a day for 1 week to 4 months and developed signs such as jaundice and itching, who improved after stopping. Some experts advise against use in pregnancy, and because ashwagandha might raise testosterone, ODS says it might not be safe for men with hormone-sensitive prostate cancer. Denmark banned ashwagandha in 2023, and France's food safety agency advised against it for some groups, including people with endocrine disorders.
Product strength and plant part vary, and ODS notes that some reports involved combination products whose contents were not checked. Useful notes for your prescriber: the product and dose, start date, your levothyroxine dose, and any palpitations, shakiness or trouble sleeping. Dose Track keeps supplements, doses and TSH results on one timeline.
Read more on Dose Track: Ashwagandha guide · Levothyroxine guide · Synthroid side effects · TSH (glossary) · Sleep