It really depends on your current vitamin D levels so best is to do the test and based on the results determine the right dosage for you. For maintenance during winter months 2000-4000IU is completely safe to take. If you are severely deficients you may even go up to 8000IU short term. If you decide to supplement vitamin D always take it with vitamin K2, magnesium and vitamin A as these are important co factors. Click here to read more about other supplements that can help reverse Hashimoto's naturally