Synthroid (Levothyroxine) Side Effects & Adverse Reactions (List) Below is a list of side effects and adverse reactions associated with synthroid. Understand that the severity and number of side effects you experience will be subject to individual variation and dose-dependent. Side effects Treatment with Synthroid typically produces few side effects, which are usually associated with allergies. The dosage is correctly determined by the doctor and is observed by the patient. Diarrhea is found among people who take Synthroid, especially for people who are female, 60+ old , have been taking the drug for 1 month, also take medication Lasix, and have High blood pressure. This study is created by eHealthMe based on reports of 161,620 people who have side effects when taking Synthroid from FDA, and is updated regularly. Along with its needed effects, levothyroxine (the active ingredient contained in Synthroid) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention. Synthroid Side Effects Synthroid is a synthetic hormone that is used to treat hypothyroidism by replacing the hormones that are not being produced due to a malfunctioning thyroid gland. Typically, this medication will be prescribed for life, so your prescription will alter as your condition changes and as you age. Find information about common, infrequent and rare side effects of Synthroid Oral. Find patient medical information for Synthroid Oral on WebMD including its uses, side effects and safety, interactions, pictures, warnings and user ratings. Hi, yes it does. Diarrhea, vomiting, or abdominal cramps (stomach cramps), These are serious side effects, contact your Doctor ASAP. from a caring individual. Synthroid may speed up chemical reactions in the gastrointestinal system that can cause diarrhea in certain patients, reports Drugs.com. A patient experiencing diarrhea may have poor absorption of food, and diarrhea may interfere with daily activities. Our Synthroid Side Effects Drug Center provides a comprehensive view of available drug information on the potential side effects when taking this medication. This is not a complete list of side effects and others may occur.