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Levothyroxine: What to expect day 1, week 1, month 1 and beyond

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About levothyroxineDay 1Days 3–5Weeks 1–3Weeks 4–6
When you receive a hypothyroidism diagnosis, your doctor will likely prescribe levothyroxine. Learn how it works, how to control your condition, and when your symptoms will improve.
Medically reviewed by Alisha D. Sellers, BS Pharmacy, PharmD
Written by Faye Stewart
Updated on

Your thyroid plays an essential role in your metabolism. The butterfly-shaped gland at the base of your neck makes hormones that affect your heart, muscle, and digestive function. It even regulates your body temperature.

When your thyroid doesn’t make enough of these hormones, you might receive a hypothyroidism diagnosis. Symptoms of hypothyroidism can include feeling cold, constipation, fatigue, weight gain, and dry skin and hair. It can also lead to mental health conditions, heart conditions, and fertility difficulties.

About levothyroxine

The gold-standard treatment for diagnosed hypothyroidism is levothyroxine (Synthroid, Levoxyl). “This is a synthetic hormone,” explains Philip K. King, PharmD. King is an assistant professor of pharmacy practice at Butler University College of Pharmacy and Health Sciences in Indianapolis. “That means it’s designed to walk and talk just like the thyroid hormone that’s created in your body. It’s chemically identical,” King explains.

The medication gives your body more of the hormone thyroxine (T4). When taken in proper doses, it will help return your thyroid hormone to optimal levels.

“Several medications can affect the absorption of levothyroxine and reduce its effects,” says Kristy L. Brittain, PharmD. Brittain is an associate professor at the Medical University of South Carolina College of Pharmacy. So make sure your doctor and pharmacist know about all your medications and over-the-counter supplements.

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Day 1

Woman's hands holding small assortment of pills above food prep area in kitchen
Photography by VorDa/Getty Images

Understanding what happens in the body after you take levothyroxine is helpful. Once the medication enters your bloodstream, the T4 hormone is converted into T3 (triiodothyronine). “This is the “active” form of the hormone that does the work,” says King.

However, this medication can be tricky. “There are a lot of things that alter the absorption of levothyroxine,” says Brittain.

You should take it on an empty stomach first thing in the morning, with a full glass of water. Then wait 30–60 minutes before eating or drinking anything else, including coffee. Otherwise, your body won’t receive most of the thyroid hormone from the medication.

“Calcium and iron can bind to levothyroxine and carry it from the body before it’s had a chance to do any work,” says Brittain. This is true whether the minerals are in food or from a supplement. “Aim to take supplements at least 4 hours after levothyroxine,” she says, “and it’s best to avoid eating calcium-rich foods, such as yogurt or milk, for breakfast.”

Antacids that contain calcium carbonate, aluminum, or magnesium can also affect absorption.

Days 3–5

“During days 3–5, blood levels of the thyroid hormone will start to rise,” says Brittain. “That will trigger your metabolism to work at a more normal pace.”

“That said, you won’t feel different yet,” adds King.

Other changes can include more stable blood sugar, improved body temperature regulation, and a less sluggish digestive system.

Here are the most common side effects you may notice when you take levothyroxine:

  • irregular heartbeats
  • shortness of breath
  • muscle cramps, pain, or weakness
  • increased appetite
  • feeling hot
  • feeling depressed or irritable
  • diarrhea

Weeks 1–3

During this time, you should start monitoring your symptoms. “In most situations, symptoms will not change for 2–3 weeks,” says King. “That’s important to know because you might not feel much different for a while,” he says.

If you notice rapid improvement within 1 week, your dose may be too high. “Early improvement of symptoms isn’t necessarily a good sign,” says King. Taking too much levothyroxine can cause the thyroid to become overactive (hyperthyroidism), and this comes with a new set of symptoms.

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Weeks 4–6

“This is when you can expect peak effects to hit,” says Brittain. “That means you’ll likely feel improvement in your symptoms. That said, some people don’t have any symptoms when they have a borderline underactive thyroid. So medication may not impact how you feel,” she adds.

Generally, the 4- to 6-week mark is when a doctor or healthcare professional will want to repeat bloodwork to check thyroid levels. Your dosing may need an adjustment to get it right,” says Brittain.

If you started on levothyroxine, your doctor likely won’t switch you to a brand-name medication. “This is one drug that we recommend patients stay on long term, as there can be minor differences between the different products, and different formulations may affect absorption,” says Brittain.

A healthcare team will monitor you until your thyroid hormones reach optimal levels. Be sure to keep them updated on changes to your morning routine, like giving up coffee or adding yogurt to breakfast, as they may need to adjust your dosage accordingly.

King notes that you might be able to take levothyroxine at night, as long as it’s 3–4 hours after your last meal or snack. That’s when your stomach will be empty again. However, you’ll still have to take it with a full glass of water.


Levothyroxine is a prescription medication that healthcare professionals prescribe for an underactive thyroid.

You should notice the effects after around 4–6 weeks, as anything sooner can mean your dosage is too high.

Working with a healthcare team is important, and letting them know about any changes to your morning routine, including what you are having for breakfast, can help them monitor your dosage best.

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