[Last updated 26th October, 2019]
Levothyroxine is the preferred medication of choice for treating an underactive thyroid (hypothyroidism).
It’s typically taken for a long period of time, if not for life, so there are many important things you should know.
This article looks at how it works, how much you need, and your alternatives if it doesn’t seem to be working.
What is Hypothyroidism?
Hypothyroidism refers to an underactive thyroid.
It can result from insufficient thyroid hormone production or because thyroid hormones are inactivated before they have time to work (1).
Left untreated it can cause many health issues including fatigue, hair loss, poor memory and weight gain.
Summary: In hypothyroidism, the level of active thyroid hormones is lower than what is needed to maintain good health.
What is Levothyroxine Sodium?
Levothyroxine sodium is an oral medication used to treat — not cure — hypothyroidism (2).
Typically just referred to as Levothyroxine, it’s a synthetic drug that is biologically identical to T4, a prohormone produced by the thyroid (2, 3).
Some of the trade names for levothyroxine sodium include (4):
- Levothroid ®
- Levo-T ®
- Levoxyl ®
- Synthroid ®
- Tirosint ®
- Unithroid ®
Summary: Levothyroxine is a synthetic prohormone that is used to treat hypothyroidism.
How Levothyroxine Works
Levothyroxine is taken by mouth and then absorbed in the small intestine (5).
Once in the body, levothyroxine (T4) is converted to triiodothyronine (T3), the most active thyroid hormone (6).
Thyroid hormones increase basal metabolic rate and influence DNA transcription, protein synthesis and metabolism (7).
Symptoms of hypothyroidism may reduce after several weeks of usage. This means a reduction in symptoms such as fatigue, cold, weight gain, hair loss and impaired memory.
Thyroid-related blood work such as TSH levels will also return to normal (9).
Summary: Taken by mouth, levothyroxine is absorbed in the small intestine and then converted to the biologically active thyroid hormone T3. Thyroid hormones influence several biochemical reactions in the body, including metabolic rate.
When to Use Levothyroxine
Levothyroxine treats several different types of hypothyroidism including (3):
- Adult hypothyroidism
- Congenital hypothyroidism
- Pregnancy-related hypothyroidism
Blood work can detect hypothyroidism even when symptoms are not present. This condition is called subclinical hypothyroidism.
Some experts recommend not prescribing levothyroxine to treat subclinical hypothyroidism because of the high risk of serious side effects (8).
Summary: Levothyroxine is used to treat hypothyroidism in children, adults and pregnant women. It’s often not prescribed in cases of subclinical hypothyroidism, when no symptoms are present.
Levothyroxine dosage depends on the patient.
For example the dose is typically lower in older patients than a middle-aged adult.
As a ballpark range, a healthy adult with no other medical issues weighing around 155 lbs (70 kg) would usually require a dosage of 100 to 150 micrograms per day.
In saying that, there are patients who can stabilize on 25-50 micrograms per day, and patients who require over 200 micrograms per day.
The dose has to be adjusted based on blood levels, which is why your doctor must determine what you need. Typically it takes a few adjustments with your doctor before the dosage will be just right.
Summary: Levothyroxine dosage varies between individuals, depending on age, weight and other medical issues. You must see your doctor to determine the best dose for you.
Levothyroxine Side Effects
Most side effects from levothyroxine stem from doses that are higher than necessary (2).
Excessively high doses can occur due to levothyroxine’s narrow therapeutic window. This means there is a small difference between having too little and too much levothyroxine.
However, when dosed correctly, the following side effects are less likely to occur (10):
- Weight loss
- Stomach cramps
- Excessive sweating
- Increased appetite
- Changes in menstrual cycle
- Sensitivity to heat
- Temporary hair loss, particularly in children during the first month of therapy.
Some of the more concerning levothyroxine side effects include (10):
- Chest pain
- Irregular heart beat
- Swelling of the ankles
- Allergic reaction to “fillers” found in levothyroxine tablets can lead to swelling of the face, tongue or throat.
Summary: If using too high a dose, levothyroxine can cause numerous side effects. In some cases, levothyroxine can result in serious cardiac, allergy and neurological symptoms.
Levothyroxine should be used carefully or not at all in some circumstances.
It should not be used as a fat loss aid by people with normal thyroid function. This can cause serious side effects (6).
Use levothyroxine with caution if you have known heart disease, are elderly, or have had severe, untreated hypothyroidism for a long time (11).
Summary: Only use levothyroxine as prescribed. Do not self-medicate in the hopes of losing weight.
Drug and Food Interactions with Levothyroxine
Levothyroxine can interact with both food and other medications.
For this reason it should be taken on an empty stomach in the morning or evening. No food should be eaten one hour either side of when it was taken.
Also note that foods like soybeans, walnuts and dietary fiber reduce its absorption (12, 13).
Levothyroxine also has significant interactions with some medications, including (6, 10):
Levothyroxine can increase the level of blood thinners, resulting in a greater risk of bleeding.
It may also decrease the level of certain heart medications such as digoxin, which can cause impaired cardiac function (14).
Summary: Be sure to discuss drug and food interactions with your physician and pharmacist before starting levothyroxine.
What if Levothyroxine Doesn’t Work?
Levothyroxine is considered the preferred treatment for hypothyroidism (15).
However, up to 10 to 20% of patients with hypothyroidism do not have a complete response to levothyroxine. This may be due to (16):
- Missed doses
- Diseases that interfere with levothyroxine absorption (e.g., celiac disease)
- Impact of other autoimmune diseases associated with hypothyroidism (e.g., type I diabetes and lupus) (17, 18).
- Drug interactions
Due to the risk of toxicity, doctors will not prescribe higher doses of levothyroxine.
T3 Combination or Desiccated Thyroid
When the response to levothyroxine isn’t enough, doctors may consider combining it with L-triiodothyronine (a synthetic form of T3).
This is considered an experimental therapy though (18,19).
Another option is desiccated thyroid hormone extract (DTE), a natural product derived from the thyroid glands of cows or pigs (20).
In one study of DTE, 48.6% of the 70 adult patients preferred how they felt on that medication as compared to levothyroxine. The DTE group also demonstrated an average weight loss of three pounds (21).
However, excessive amounts of T3 have been found in some batches of desiccated thyroid, raising concerns about its use.
Further, desiccated thyroid has not been proven to be as safe or effective as levothyroxine in the treatment of hypothyroidism. As a result, levothyroxine remains the drug of choice for managing an underactive thyroid. (9, 22).
But you can always request desiccated thryoid with your doctor if levothyroxine is not working for you.
Summary: Levothyroxine is the treatment of choice for hypothyroidism. Other treatment options, including the use of T3 or desiccated thyroid hormone are not recommended.
Levothyroxine is a synthetic prohormone proven to be effective in the management of hypothyroidism.
It reduces symptoms by increasing thyroid hormone levels and research indicates it’s superior to other treatments like desiccated thyroid hormone.
However, due to the potential for toxicity and multiple food and drug interactions, be sure to carefully follow your doctor’s instructions.