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Levothyroxine is a synthetic (lab-made) version of the natural thyroid hormone thyroxine or T4. It is a prescribed medication that is used to replace thyroid hormone in people with hypothyroidism (an underactive thyroid gland), a lifelong condition where the thyroid gland doesn’t make enough thyroid hormones. Levothyroxine starts working right away after you take the first dose, but it can be several weeks before you notice an improvement in your hypothyroidism (underactive thyroid) symptoms. Please continue reading to learn more.

How long after starting levothyroxine will I feel better?

Levothyroxine starts working right away after you take the first dose. However, it can take a few weeks before you start to feel better and notice significant changes in your symptoms. You may start to notice some improvement in your hypothyroidism symptoms after one week and feel much better after 2-3 weeks. The peak effects are generally around 4 to 6 weeks after starting treatment. Blood work is typically ordered around this time to ensure the medication’s effectiveness. 

Continue taking levothyroxine even if you don’t feel any different initially. Do not discontinue levothyroxine treatment without talking to your healthcare provider. Also, do not switch from one brand to another without your doctor’s approval. Your provider may need to adjust your dose a few times before finding the right dose. 

How long does it take for thyroid levels to stabilize?

It takes 4-6 weeks for thyroid levels to stabilize. Once you have been diagnosed with hypothyroidism and started on levothyroxine therapy, your doctor will order a blood test called thyroid stimulating hormone (TSH) to monitor your response to thyroid hormone replacement and adjust your dose.

How does levothyroxine help with an underactive thyroid gland?

Levothyroxine is a prescription medication that is used to treat hypothyroidism (underactive thyroid gland). The generic drug levothyroxine is also sold under brand names such as Synthroid, Levoxyl, Unithroid, Tirosint, and Levo-T. Check out our blog to learn the difference between an underactive vs overactive thyroid.

The thyroid gland is a small gland in the neck that makes thyroid hormones. These hormones control many metabolic processes in the body, play an essential role in growth and development, and support the nervous system and immune function. Certain thyroid disorders, such as hypothyroidism (underactive thyroid), are associated with low levels of thyroid hormones. Taking levothyroxine can help to normalize thyroid hormone levels and relieve the symptoms of hypothyroidism. Check out 8 signs you may have a thyroid condition.

Treatment with levothyroxine also improves other health problems, for instance, by making it easier to lose weight and improving cholesterol levels, blood pressure, and heart rate, lowering the risk of heart disease.

How do I know if levothyroxine is working?

You will know levothyroxine is working when your body responds to the medicine and your hypothyroidism symptoms start to improve after a little while. 

Untreated hypothyroidism can cause you to experience symptoms such as fatigue, low energy levels, slowed heart rate, weight gain, cold intolerance, joint pain, muscle pain, dry skin, hair loss, thinning hair, fertility problems, heavy or irregular menstrual periods, and depression. Treatment with levothyroxine medication replaces thyroid hormone in your body and improves these symptoms. Check out our blog to find out: “What are the signs my thyroid medication is not working?”

What is thyroid stimulating hormone (TSH)?

Thyroid-stimulating hormone (TSH) is a hormone made by the pituitary gland in the brain. Blood levels of TSH tell your doctor how your body is responding to levothyroxine. When thyroid hormone levels in the body are low, the pituitary gland releases more TSH. Therefore, high TSH test results tell your doctor that your levothyroxine dose should be increased. On the other hand, if your TSH level is low, it tells your doctor that your levothyroxine dosage is high and should be reduced. 

According to the American Thyroid Association, for most patients taking levothyroxine replacement, the goal TSH level is between 0.5 to 2.5 milliunits per liter (mU/L) to achieve normal thyroid function. People who have had thyroid cancer or thyroid surgery may have a lower target range, and elderly patients may have a higher target range.

Your doctor will obtain a TSH blood test 2-3 months after starting levothyroxine and then at least every 6-12 months while you are on a stable levothyroxine dose. You may need more frequent blood tests (every 6-8 weeks) if your dosage is being adjusted. You also might need more regular blood tests if you are on levothyroxine and are currently pregnant or planning a pregnancy soon. 

What are typical levothyroxine thyroid medication levels?

Your doctor may start you on a lower dose of levothyroxine and increase it as needed based on TSH levels and side effects. Typical levothyroxine dosing is as follows:

• Adults and children 12 years of age and older (post-pubertal, growth complete): 1.7 mcg per kilogram of body weight per day. 

• Children 12 years of age and older (pre-pubertal, growth incomplete): 2-3 micrograms (mcg)/kilograms (kg)/day.

• Children 6-12 years of age: 4-5 mcg/kg/day.

Check out “Understanding levothyroxine side effects.”

Is it better to take levothyroxine in the morning or evening? 

It is better to take levothyroxine in the morning on an empty stomach, 30-60 minutes before breakfast. 

Read the prescription label carefully and make sure you take the correct dose. Taking too much levothyroxine can result in symptoms similar to a thyroid disease called hyperthyroidism (overactive thyroid), with a racing heart (palpitations), heat intolerance, and anxiety. Learn more about hypothyroidism vs hyperthyroidism. 

How to take levothyroxine?

Swallow your thyroid medication with a full glass of water. Do not crush or chew levothyroxine capsules. You can, however, crush a levothyroxine tablet and mix it with 5-10 mL (1-2 teaspoons) of water (do not mix it with infant soybean formula or food). 

If you forget to take a dose of your thyroid medication, take it as soon as you remember. However, if it is almost time for the next dose, skip the missed dose and go back to your regular dosing schedule. Do not take a double dose or extra medicine to make up for a missed dose.

What else can affect how fast levothyroxine works?

The presence of certain foods in the stomach can affect the absorption of levothyroxine and how well it works. Talk to your healthcare professional about eating grapefruit and drinking grapefruit juice while on levothyroxine. Also, avoid eating walnuts, soybeans, dietary fiber, and calcium-rich foods around the time you take your levothyroxine dose. 

In addition, other medications can affect how levothyroxine works. Before you start medication for an underactive thyroid, give your doctor a complete list of all your other medications, including prescription drugs, over-the-counter medications, dietary supplements, and herbal products. This will help avoid potential interactions with your thyroid medication. 

Medications can also interfere with laboratory test results. For example, biotin supplements (which are often present in combination therapy such as multivitamins) can interfere with the results of thyroid function tests, resulting in wrong diagnosis or wrong treatment.