Short answer: The Cytomel label says its side effects are mainly those of too much thyroid hormone (overdosage): fast or irregular heartbeat, palpitations, nervousness, insomnia, tremor, muscle cramps, heat intolerance, excess sweating, weight loss, diarrhea, headache and menstrual changes. It gives no percentages. Cytomel carries a boxed warning that thyroid hormones must not be used for obesity or weight loss.
What Cytomel is approved for: Cytomel is approved as replacement therapy for hypothyroidism from any cause (thyroid, pituitary or hypothalamic, present at birth or acquired), as an add-on to surgery and radioiodine in well-differentiated thyroid cancer, and as a diagnostic drug in thyroid suppression tests. 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
CYTOMEL (liothyronine sodium) tablets, for oral use, prescribing information. Pfizer. DailyMed, version published May 21, 2026. DailyMed setid 51452b31-ff68-4e0c-b982-c15502ebf1d3
WARNING: NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS
Thyroid hormones, including Cytomel, 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 Cytomel.
| 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 and cramps |
| 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 |
| Endocrine | decreased bone mineral density |
| Reproductive | menstrual irregularities, impaired fertility |
The label says adverse reactions with Cytomel are primarily those of hyperthyroidism due to therapeutic overdosage. It gives no percentages and no placebo comparison.
In children on thyroid replacement, pseudotumor cerebri (raised pressure around the brain) and slipped capital femoral epiphysis (a hip growth-plate problem) have been reported. Overtreatment can cause early skull-bone closure in infants and early growth-plate closure in 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 such as pain, nausea, vomiting and diarrhea, fever, joint pain, serum sickness and wheezing.
Partial hair loss may occur rarely during the first few months of Cytomel 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.
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 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, so starting it before steroid replacement can trigger an acute adrenal crisis. Adrenal insufficiency is treated with glucocorticoids first. Uncorrected adrenal insufficiency is the label's one contraindication (section 4).
Too much or too little thyroid hormone. Cytomel has a narrow therapeutic index. Over- or undertreatment can affect growth, the heart, bones, fertility, thinking, mood, digestion and blood sugar and fat levels, so the dose is titrated carefully and food and drug interactions are watched.
Worse blood sugar control in diabetes. Adding thyroid hormone may worsen blood sugar control and raise the need for diabetes medicine or insulin. Blood sugar is watched closely after starting, changing or stopping Cytomel.
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 Cytomel:
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.