Unithroid Side Effects: What to Expect and When to Call Your Doctor

Understanding Unithroid side effects is important — but it helps to understand why they occur. Most side effects associated with Unithroid (levothyroxine sodium) are not caused by the drug itself in the traditional sense. They are symptoms of too much thyroid hormone in your body (hyperthyroidism or overtreatment), which can happen when your dose is too high. When your dose is correct and your TSH is in the normal range, most patients tolerate levothyroxine very well.

Common Side Effects of Unithroid

The following side effects are most frequently reported with Unithroid, especially during the early weeks of treatment or after a dose increase:

Temporary hair loss: Thinning hair is common in the first 3-6 months of starting levothyroxine. This typically resolves on its own as your thyroid levels stabilize.

Headache:

Insomnia and sleep disturbances:

Nervousness and irritability:

Fever, hot flashes, and sweating:

Heart palpitations:

Changes in menstrual cycle:

Appetite changes and weight loss:

Diarrhea:

Heat intolerance:

If you experience these side effects, notify your doctor. Your dose may be too high and a reduction may be needed. Do not reduce your dose on your own — levothyroxine dose changes require TSH monitoring to guide adjustments.

Serious Side Effects: Call Your Doctor Right Away

These side effects are potentially serious and require prompt medical attention:

Chest pain, rapid or irregular heartbeat (arrhythmia): Overtreatment can precipitate arrhythmias or worsen existing heart disease — especially in elderly patients. Call your doctor immediately or seek emergency care.

Sudden bone or joint pain: Long-term overtreatment increases bone resorption and can reduce bone mineral density — particularly in postmenopausal women. Sudden hip, wrist, or back pain may signal a stress fracture.

Signs of allergic reaction: Hives, difficulty breathing, swelling of face, lips, tongue, or throat. Seek emergency help immediately. Note: true allergy to levothyroxine itself is rare; reactions are more commonly due to inactive ingredients (Unithroid contains lactose and dyes).

Seizures, stroke, or loss of consciousness (overdose): Levothyroxine overdose is a medical emergency. Symptoms include severe tremor, extreme agitation, chest pain, stroke, coma. Call 911 immediately.

Boxed Warning: Do Not Use for Weight Loss

Unithroid carries a boxed warning (the FDA's most serious warning level) stating that thyroid hormones should not be used for weight loss or obesity treatment. In patients with normal thyroid function (euthyroid), doses within the daily hormonal range will not cause weight loss. Larger doses may produce serious or life-threatening effects, particularly when combined with appetite suppressants or sympathomimetic amines.

Side Effects of Undertreatment (Too Low a Dose)

Undertreatment — taking too low a dose or missing doses — can also cause symptoms. These are hypothyroid symptoms that persist or return despite being on Unithroid:

Fatigue and low energy

Unexplained weight gain

Cold intolerance

Brain fog and memory problems

Constipation

Depression or low mood

Dry skin and hair

If you are experiencing these symptoms while on Unithroid, contact your doctor to get your TSH checked. Your dose may need to be increased, or you may have an absorption problem (e.g., calcium or iron supplements taken too close to your Unithroid dose).

Side Effects in Special Populations

Elderly patients: More susceptible to cardiac side effects; start at lower doses (25 mcg/day) and titrate slowly.

Pregnant patients: Higher doses are often needed. Undetected undertreatment risks fetal neurodevelopment; TSH must be monitored every 4 weeks in the first half of pregnancy.

Patients with cardiovascular disease: Risk of arrhythmia and angina with overtreatment; start at reduced doses and monitor cardiac symptoms closely.

Side effects can also be caused or worsened by drug interactions. See our guide on Unithroid drug interactions to learn what to avoid.