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Drug Info

How long does it take levothyroxine to work?

Levothyroxine starts to work right away, but it may take a few weeks for your symptoms to improve
Karen Berger medical writer and reviewer headshot
By | Jul. 3, 2024
A blue analog alarm clock and Rx tablets: How long does it take for levothyroxine to work?

Key takeaways

  • Levothyroxine, commonly known by the brand name Synthroid, is a thyroid medication used to treat hypothyroidism (underactive thyroid). The full effect on thyroid hormone levels and symptoms may take four to six weeks to see.

  • Take levothyroxine once daily on an empty stomach with a full glass of water. Many take it in the morning, 30 to 60 minutes before breakfast.

  • Certain foods and medications can affect levothyroxine absorption, so you may need to adjust the timing of your medications.

  • Keep all appointments with your healthcare provider and for blood work. Also, consult your healthcare provider if you experience symptoms of hypothyroidism or hyperthyroidism, as your dose may need adjustment.

In this article:

Levothyroxine, also known by brand names like Synthroid, Tirosint, and Levoxyl, is a thyroid medicine known as thyroid replacement therapy or thyroid hormone replacement. It is used in people with an underactive thyroid (hypothyroidism) and replaces the thyroid hormone the body normally makes. It is also used to treat certain types of thyroid cancer and thyroid disorders. Many people who start taking levothyroxine for hypothyroidism want to know how fast it will start working. The good news is that levothyroxine starts to work immediately for thyroid disease, but it may take up to four to six weeks to notice the medication’s full benefit. Continue reading to learn more about levothyroxine.

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How long does it take for levothyroxine to work? 

When your healthcare provider prescribes levothyroxine, they will determine the starting dose depending on several factors, such as your age, weight, lab values, and other medications you take. 

Levothyroxine starts to work immediately; however, it may take up to four to six weeks to see its full effect. Even if you do not notice a difference in symptoms immediately, the medication takes time to start regulating your thyroid levels. It’s important to continue to take your daily dose as directed.

A dose adjustment may be required in some cases. Your healthcare provider will order blood work, generally about six weeks after you start taking levothyroxine, to see how well the medication is working. The blood tests will check your thyroid-stimulating hormone (TSH) levels to measure your thyroid function.

Once the medication reaches a steady level in the body, in about six weeks, your hypothyroidism symptoms should subside as your thyroid hormone levels regulate. 

If, after several weeks, your symptoms worsen or do not improve, your healthcare provider may need to adjust the dose of your medicine. Generally, healthcare providers will check TSH levels and adjust doses every six to eight weeks, if required, until thyroid levels normalize. Older adults or people with heart problems may be monitored more frequently, like every four to six weeks, until levels normalize. Once your healthcare provider determines that you are taking the right dose and your TSH level is appropriate, they may check again in three to six months, then every six to 12 months. 

How do you know levothyroxine is working?

Some common symptoms of hypothyroidism include:

  • Fatigue 
  • Constipation 
  • Weight gain
  • Sore muscles
  • Depression 
  • Dry skin and hair
  • Feeling cold
  • Brain fog
  • Irregular heartbeats

If you are taking levothyroxine and you notice symptoms of hypothyroidism (low thyroid levels), consult your healthcare provider. You may need a higher dose of levothyroxine, which will normalize your TSH levels and reduce these symptoms. 

Common symptoms of hyperthyroidism (overactive thyroid) include:

  • Fast heart rate or palpitations
  • Chest pain
  • Shakiness
  • Nervousness
  • Weight loss
  • Increased appetite
  • Diarrhea
  • Thin and warm skin
  • Feeling hot; excess sweating
  • Trouble sleeping
  • Neck swelling from an enlarged thyroid gland (goiter)
  • Hair loss
  • Weak muscles
  • Bulging eyes (seen with Graves’ disease, a common cause of hyperthyroidism)

If you take levothyroxine and notice symptoms of hyperthyroidism, you should consult your healthcare provider. You may require a lower dose of levothyroxine to help normalize TSH levels and reduce symptoms. 

Side effects of levothyroxine are generally those of hyperthyroidism, which can occur when the dose is too high.

As you can see, taking your medication as directed is important, and notify your healthcare provider if you notice any underactive or overactive thyroid symptoms.

RELATED: Hypothyroidism vs. hyperthyroidism

What is the standard levothyroxine dosage?

There is no standard dose for levothyroxine, as the dosages vary based on various factors. 

Levothyroxine tablets are available in the following strengths: 

  • 25 micrograms (mcg)
  • 50 mcg
  • 75 mcg
  • 88 mcg
  • 100 mcg
  • 112 mcg
  • 125 mcg 
  • 137 mcg
  • 150 mcg 
  • 175 mcg 
  • 200 mcg 
  • 300 mcg

For those younger than 60, a general starting dose may be 1.5 to 1.8 mcg per kilogram (kg) daily. 

To translate this into a real-life example, if someone weighs 175 pounds, that equals about 80 kilograms. If the healthcare provider wants to start with 1.5 mcg per kg daily, they would multiply 1.5 x 80, and the answer would be 120. 

Considering the available doses of levothyroxine above, the starting dose would be either 112 mcg or 125 mcg.

For older adults or people with heart disease, healthcare providers generally follow the guidelines from 12.5 mcg to 50 mcg daily. 

Tips for taking levothyroxine

Here are some tips for taking levothyroxine:

  • Before taking levothyroxine, tell your healthcare provider about any health conditions you have, including if you are pregnant, planning to become pregnant, or breastfeeding.
  • Take levothyroxine exactly as directed by your healthcare provider. Take only what is prescribed. 
  • Take levothyroxine once daily on an empty stomach, 30 to 60 minutes before eating. Many patients take their daily dose before breakfast. Take your medicine with a full glass of water to avoid choking or gagging.
  • See the next section for more information about the timing of certain other foods and medications.
  • It may take several weeks until you notice your symptoms subside. Continue to take your medicine as directed.
  • If you are scheduled for any type of surgery, tell your healthcare provider that you take levothyroxine. 

What can affect how long it takes levothyroxine to work?

Certain factors can affect the absorption of levothyroxine, such as the timing of medication, other foods, and other medications or supplements.

Timing of medication 

Levothyroxine is best absorbed on an empty stomach. Many people take levothyroxine in the morning, 30 to 60 minutes before breakfast, as the prescribing information recommends. 

Some individuals, though, prefer to take levothyroxine at night. That is generally fine, as long as you take it at least three or four hours after your last meal of the day so that you take it on an empty stomach.

It’s also best to take levothyroxine at about the same time every day. 

Foods 

Certain foods or ingredients can interfere with the absorption of levothyroxine, including:

  • Soybean flour
  • Cottonseed meal
  • Walnuts
  • Dietary fiber
  • Grapefruit and grapefruit juice

If you consume anything on this list, consult your healthcare provider. Avoiding these foods or ingredients within an hour or two of taking your medication should suffice.

Medications or supplements

Various medications, including certain prescription drugs, OTC meds, and dietary supplements, can interfere with the absorption of levothyroxine. If you take anything on the list below, avoid taking them for four hours before or four hours after taking levothyroxine:

  • Antacids such as Maalox, Mylanta, Milk of Magnesia
  • Bile acid sequestrants such as Colestid (colestipol), Questran (cholestyramine), or Welchol (colesevelam)
  • Calcium products such as Tums or calcium vitamins
  • Carafate (sucralfate)
  • Iron supplements
  • Kayexalate (sodium polystyrene sulfonate)
  • Phosphate binders such as Fosrenol (lanthanum), Renagel (sevelamer), or Renvela (sevelamer)
  • Proton pump inhibitors such as Nexium (esomeprazole) or Protonix (pantoprazole)

Other medications may affect levothyroxine as well. Consult your healthcare professional for more information and medical advice. 

The bottom line

If you are taking levothyroxine for hypothyroidism, it may take up to six weeks to notice the full benefit of the medication. Take your medication once daily as directed, on an empty stomach—and always with a full glass of water to prevent choking or gagging. Your healthcare provider will tell you when to follow up for blood work, and you should consult your provider if you notice any underactive or overactive thyroid symptoms, which may require a dosage adjustment.

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