When your thyroid doesn’t produce enough hormone, levothyroxine, a synthetic form of the thyroid hormone T4 used to treat hypothyroidism. Also known as synthroid, it’s one of the most commonly prescribed medications in the world. Getting the levothyroxine dose right isn’t just about numbers—it’s about how your body responds. Too little and you’ll still feel tired, cold, and sluggish. Too much and you might get heart palpitations, trouble sleeping, or even bone loss over time.
The starting dose isn’t the same for everyone. A 25-year-old with mild hypothyroidism might begin at 50 mcg, while a 70-year-old with heart issues could start at 25 mcg or even less. Doctors adjust based on age, weight, other health conditions, and blood test results—especially TSH levels. It can take weeks for your body to fully respond, so don’t expect instant changes. Many people need at least 6–8 weeks before their dose is fine-tuned. And don’t skip follow-up tests. Thyroid needs can shift over time, especially during pregnancy, after weight loss, or as you age.
TSH, thyroid-stimulating hormone produced by the pituitary gland to regulate thyroid function is the main guide for dosing. If TSH is high, your dose is likely too low. If it’s too low, you might be overmedicated. But TSH isn’t the whole story. Some people still feel unwell even with normal TSH—this is where symptoms and free T4 levels matter too. Hypothyroidism, a condition where the thyroid gland doesn’t make enough hormones, leading to slowed metabolism and fatigue affects people differently. Some need exactly what the labs say. Others need a little more or less to feel normal.
Timing and what you eat matter more than you think. Levothyroxine works best taken on an empty stomach, at least 30–60 minutes before breakfast. Coffee, calcium, iron, and even soy can block absorption. Taking it at bedtime can work too—if you haven’t eaten for 3–4 hours. Consistency is key. Taking it at the same time every day helps keep levels steady. And never switch brands without checking with your doctor. Even though generics are chemically the same, small differences in fillers can affect how your body absorbs the drug.
Side effects are rare when the dose is right. But if you’re suddenly anxious, losing weight without trying, or having chest pain, talk to your doctor. These aren’t normal. And don’t stop or change your dose on your own—even if you feel better. Stopping suddenly can crash your metabolism and bring back all the worst symptoms. Many people stay on levothyroxine for life, but that doesn’t mean it’s a one-size-fits-all treatment. It’s a personalized tool, adjusted over time to match your changing body.
Below, you’ll find real-world insights from patients and providers on how levothyroxine works in practice—what dosing mistakes to avoid, how other meds interact with it, and why some people need more than just a number on a prescription.
Thyroid medication doses must increase by 20-30% during pregnancy to support fetal brain development. TSH should be kept under 2.5 mIU/mL in the first trimester, with testing every 4 weeks. Levothyroxine is safe and essential - delaying adjustments can impact your child's IQ.
