What Is Synthroid? Uses, Dosage, and What You Need to Know in 2026

Synthroid is one of the most prescribed medications in the United States — yet many patients who take it daily aren't entirely sure what it does, how it works, or why they need it. If you've been prescribed Synthroid and want to understand it better, this guide covers everything you need to know.

What Is Synthroid?

Synthroid is the brand name for levothyroxine sodium — a synthetic version of the thyroid hormone T4 (thyroxine) that your thyroid gland normally produces. It is made by AbbVie and is classified as a thyroid hormone replacement drug.

Levothyroxine is available as a generic and under several brand names, including Synthroid, Levoxyl, Tirosint, Euthyrox, and Unithroid. Synthroid is the most prescribed branded version.

What Is Synthroid Used For?

Synthroid has two FDA-approved uses:

Hypothyroidism (underactive thyroid): Replaces thyroid hormone that the thyroid gland isn't producing enough of. This includes primary hypothyroidism (thyroid gland problem), secondary hypothyroidism (pituitary gland problem), and tertiary hypothyroidism (hypothalamus problem). The most common cause is Hashimoto's thyroiditis, an autoimmune condition.

Thyroid cancer management: Used as an adjunct to surgery and radioiodine therapy to suppress TSH (thyroid-stimulating hormone), which can stimulate growth of remaining cancer cells. This is called TSH suppression therapy.

What Synthroid is NOT for: weight loss. The FDA includes a boxed warning against using Synthroid or any thyroid hormone for weight loss purposes.

Symptoms Synthroid Treats

Hypothyroidism causes a wide range of symptoms that Synthroid helps resolve:

Fatigue and sluggishness

Weight gain

Cold sensitivity

Constipation

Dry skin and hair

Depression and brain fog

Slow heart rate

Muscle weakness or joint pain

Irregular menstrual periods

Synthroid Dosage: How Is It Dosed?

Synthroid comes in 12 different strengths: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg. The typical adult starting dose is calculated as 1.6 mcg/kg of body weight per day. For someone weighing 70 kg (154 lbs), that's approximately 112 mcg/day.

Starting doses are often lower — especially for elderly patients or those with heart disease — typically beginning at 25–50 mcg/day and increasing gradually every 4–8 weeks based on TSH monitoring.

Dosage in children varies by age: neonates need 10–15 mcg/kg/day, 1-year-olds need 4–6 mcg/kg/day, adolescents need 2–4 mcg/kg/day. Pediatric doses are adjusted to maintain TSH and T4 in the appropriate ranges for age.

How to Take Synthroid

Take it first thing in the morning, on an empty stomach, 30–60 minutes before eating or drinking anything except water

Take it at the same time every day — consistency matters

Swallow the tablet with a full glass of water

Wait at least 4 hours before taking calcium supplements, iron, antacids, or certain other medications that reduce absorption

Avoid coffee within 15 minutes of your dose — it can reduce absorption by up to 27%

Is Synthroid a Lifelong Medication?

For most patients, yes. Hypothyroidism caused by Hashimoto's thyroiditis, thyroid removal, or radioactive iodine treatment is permanent and requires lifelong thyroid hormone replacement. Some patients with transient hypothyroidism (e.g., from subacute thyroiditis) may be able to stop treatment — but this is determined by your doctor through blood work, not by stopping on your own.

Cost and Savings

Generic levothyroxine costs as little as $9.90/month with discount coupons. Brand-name Synthroid runs $40–$80/month cash-pay at retail, but AbbVie's Synthroid Delivers Program offers it for $39.95/month by mail. See our full Synthroid savings guide for all your options.

If you're having trouble finding Synthroid at your pharmacy, medfinder can help you locate it in stock near you.