Levothyroxine is generally well tolerated, but it can cause side effects ranging from mild and temporary to serious. Most side effects stem not from the drug itself but from getting the dose wrong. Too much levothyroxine essentially mimics an overactive thyroid, producing symptoms like a racing heart, anxiety, and weight loss. Too little, and your original hypothyroid symptoms persist. The goal is a dose that keeps your thyroid hormone levels in a narrow, normal range.
Common Side Effects
The most frequently reported side effects of levothyroxine include headache, changes in appetite, weight fluctuations, diarrhea, and sensitivity to heat. Some people also experience joint pain, leg cramps, fever, or changes in their menstrual cycle. These effects often show up when you first start the medication or after a dose change, and many resolve as your body adjusts over several weeks.
Hair loss deserves a special mention because it catches people off guard. Partial hair thinning is common during the first month of treatment, especially in children. This happens because shifting thyroid hormone levels temporarily disrupt the hair growth cycle. Once your levels stabilize, hair growth typically returns to normal within a few months.
Signs Your Dose Is Too High
The most common reason for levothyroxine side effects is overtreatment: your dose is pushing thyroid hormone levels higher than your body needs. Mild overtreatment feels like feeling hot or shaky, having trouble falling asleep, heart palpitations, and excessive sweating.
More severe overtreatment produces a wider constellation of symptoms: anxiety, mood swings, hand tremors, diarrhea, muscle weakness (particularly in the thighs and shoulders), unexplained weight loss, inability to concentrate, forgetfulness, and a resting heart rate that feels abnormally fast. If you notice any of these, it usually means your dose needs to come down rather than that you need to stop the medication entirely. A blood test measuring your TSH level will confirm whether the dose is off.
Heart-Related Risks
Levothyroxine’s most concerning side effects involve the heart, particularly in older adults. Chest pain, a racing or irregular heartbeat, shortness of breath, and pain radiating to the arms, jaw, or neck all warrant immediate medical attention.
The connection between excess thyroid hormone and irregular heart rhythms is well documented. Data from the Framingham Heart Study found that among people over 60 with mildly suppressed TSH levels (a sign of too much thyroid hormone, whether from the gland itself or from medication), 28% developed atrial fibrillation over a ten-year period, compared to 11% of those with normal levels. The risk scales with how suppressed the TSH becomes. This is why regular monitoring matters so much, especially for older adults.
Effects on Bone Density
Thyroid hormone directly affects bone. When levels run too high for too long, bone breaks down faster than it rebuilds, leading to reduced bone density and a higher fracture risk. According to the American Thyroid Association, levothyroxine therapy that suppresses TSH below normal has been associated with bone loss and osteoporosis.
The reassuring finding is that this only applies to overtreatment. When levothyroxine doses are calibrated to maintain a normal TSH, there is no measurable effect on bone density. This is another reason your dose should be the lowest amount that normalizes your levels, not more.
Side Effects in Children
Children on levothyroxine face a few unique concerns. Overtreatment can accelerate bone maturation, causing growth plates to close prematurely and potentially compromising adult height. In infants, excessive doses have been linked to premature fusion of skull bones. There have also been rare reports of a condition called pseudotumor cerebri, which causes increased pressure inside the skull and can produce severe headaches and vision changes.
Children with long-standing or severe hypothyroidism are more likely to experience irritability, restlessness, decreased attention span, and insomnia when treatment begins. Doctors typically start children on a low dose and increase it gradually over several weeks to minimize these effects.
Reactions to Inactive Ingredients
True allergic reactions to levothyroxine itself are essentially unheard of. When people do have hypersensitivity reactions, like hives, skin rash, flushing, stomach pain, or wheezing, the culprit is almost always one of the inactive ingredients in the tablet. Levothyroxine formulations contain fillers including lactose, corn starch, acacia, and various color dyes that differ by tablet strength. The 25 mcg tablet contains a yellow dye, the 150 mcg tablet contains a blue dye, and so on through the full range of doses.
If you suspect a reaction to a filler or dye, switching to a different manufacturer’s formulation or to a dye-free option can often solve the problem. Gel capsule and liquid forms of levothyroxine contain different inactive ingredients and may be better tolerated.
Supplements That Interfere With Absorption
Some side effects aren’t caused by levothyroxine itself but by poor absorption that leads to unstable hormone levels. Calcium supplements, antacids, and iron supplements all bind to levothyroxine in the gut and prevent it from being absorbed properly. You should take any calcium or iron-containing products at least four hours before or after your levothyroxine dose. The standard recommendation is to take levothyroxine on an empty stomach, 30 to 60 minutes before eating.
Inconsistent absorption leads to fluctuating thyroid levels, which can cycle you between symptoms of undertreatment and overtreatment. If your levels seem hard to stabilize despite regular dosing, reviewing what you take alongside levothyroxine is a practical first step.
How Monitoring Helps Prevent Side Effects
Most levothyroxine side effects are dose-dependent, which means they’re preventable with proper monitoring. After starting the medication or changing your dose, a TSH blood test is typically done at six weeks. From there, testing repeats every three months until levels are stable, then usually once or twice a year for ongoing maintenance.
The six-week window matters because thyroid hormone levels take that long to reach a new steady state in your body. Testing too early can give misleading results and lead to unnecessary dose changes, which only prolongs the adjustment period and the side effects that come with it.
Overdose Warning Signs
A significant overdose of levothyroxine is a medical emergency. Symptoms include confusion, disorientation, loss of consciousness, cold and clammy skin, a fast or weak pulse, sudden loss of coordination, sudden slurring of speech, and severe headache. These symptoms require emergency medical care immediately.