Short answer: The Synthroid label says its side effects are mainly those of too much thyroid hormone (overdosage): fast or irregular heartbeat, palpitations, nervousness, trouble sleeping, tremor, heat intolerance, excess sweating, weight loss, diarrhea, headache and changes in menstrual periods. It gives no percentages. Synthroid has a boxed warning that it must not be used for weight loss or obesity.
What Synthroid is approved for: Synthroid is approved for adults and children, including newborns, as replacement therapy for hypothyroidism from any cause (thyroid, pituitary or hypothalamic, present at birth or acquired), and as an add-on to surgery and radioiodine to suppress TSH in TSH-dependent well-differentiated thyroid cancer. It is not approved to suppress benign thyroid nodules or nontoxic goiter in people with enough iodine, or for hypothyroidism during recovery from subacute thyroiditis (section 1).
Source
SYNTHROID (levothyroxine sodium) tablets, for oral use, prescribing information. AbbVie. DailyMed, version published February 29, 2024. DailyMed setid 1e11ad30-1041-4520-10b0-8f9d30d30fcc
WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS
Thyroid hormones, including Synthroid, should not be used to treat obesity or for weight loss, alone or with other drugs. In people with normal thyroid function, usual doses do not cause weight loss, and larger doses can cause serious or even life-threatening toxicity, especially when combined with stimulant appetite suppressants.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Synthroid.
| Body system | Reactions the label lists |
|---|---|
| General | fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating |
| Central nervous system | headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia |
| Musculoskeletal | tremors, muscle weakness, muscle spasm |
| Cardiovascular | palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest |
| Respiratory | dyspnea |
| Gastrointestinal | diarrhea, vomiting, abdominal cramps, elevations in liver function tests |
| Dermatologic | hair loss, flushing, rash |
| Endocrine | decreased bone mineral density |
| Reproductive | menstrual irregularities, impaired fertility |
The label says adverse reactions with Synthroid are primarily those of hyperthyroidism due to therapeutic overdosage. It gives no percentages and no placebo comparison.
Seizures have been reported rarely when levothyroxine is started.
In children, pseudotumor cerebri (raised pressure around the brain) and slipped capital femoral epiphysis (a hip growth-plate problem) have been reported. Overtreatment can cause early closure of the skull bones in infants whose soft spots have not closed, and early closure of growth plates in growing children, which can reduce adult height.
Allergic-type reactions to the inactive ingredients have occurred with thyroid hormone products: hives, itching, rash, flushing, swelling (angioedema), stomach symptoms, fever, joint pain, serum sickness and wheezing. The label says allergy to levothyroxine itself is not known to occur.
Partial hair loss may occur rarely during the first few months of Synthroid therapy, and the label says it is usually temporary.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Too much or too little thyroid hormone. Synthroid has a narrow therapeutic index, so small dose differences matter. Overtreatment or undertreatment can affect growth, the heart, bones, fertility, thinking, digestion and blood sugar and fat levels. In children, undertreatment can harm brain development and growth, and overtreatment is linked to early skull-bone closure and faster bone aging.
Heart effects in older people and people with heart disease. Overtreatment can raise heart rate and heart muscle thickness and strength of contraction, and may bring on chest pain (angina) or irregular heartbeat. These people start at lower doses. If heart symptoms develop or worsen, the label says the dose is reduced or held for one week and restarted lower.
Myxedema coma. Myxedema coma is a life-threatening emergency in which absorption of tablets from the gut is unpredictable. The label says oral thyroid hormone is not recommended for it; intravenous thyroid hormone is used instead.
Adrenal crisis with untreated adrenal insufficiency. Thyroid hormone speeds the clearance of steroid hormones. Starting it before steroid replacement can trigger an acute adrenal crisis, so adrenal insufficiency is treated with glucocorticoids first. Uncorrected adrenal insufficiency is the label's one contraindication (section 4).
Worse blood sugar control in diabetes. Adding levothyroxine may worsen blood sugar control and raise the need for diabetes medicine or insulin. Blood sugar is watched closely after starting, changing or stopping Synthroid.
Bone loss from too much thyroid hormone. Over-replacement can increase bone breakdown and lower bone mineral density, especially in post-menopausal women. The label says to use the lowest dose that gets the desired clinical and lab response.
From the Patient Counseling Information (section 17); the label has no separate Medication Guide that comes with Synthroid:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.