Levothyroxine: Drug Class, Effects, & Nursing Considerations

Welcome students or nursing students to another nursing class informative post about levothyroxine. This lesson will provide information on this medication’s dosing, contraindications, side effects, and other essential aspects.

What is Levothyroxine?

Levothyroxine, also known as L-thyroxine, is a medication used to treat thyroid hormone deficiency for several decades. When the body cannot produce enough of the thyroid hormone thyroxine, it can result in a wide range of symptoms, including fatigue, weight gain, and depression.

However, thanks to the development of this synthetic version of the hormone, those suffering from hypothyroidism can now find relief from their symptoms. Additionally, levothyroxine is sometimes used to treat and prevent certain thyroid tumors. This medication can be a lifesaver for those who struggle with thyroid issues.

Levothyroxine is a medication that has been improving people’s lives for over nine decades. Made for the first time in 1927, it has become an essential medicine listed by the World Health Organization. As a generic medication, it is accessible to many. It has helped millions of people struggling with thyroid problems- a common issue where the thyroid gland produces insufficient hormones.

It is not surprising that Levothyroxine was the second most commonly prescribed medication in the United States in 2020, with over 98 million prescriptions filled. Its effectiveness, combined with accessibility, has made Levothyroxine a staple in the world of medicine.

Brand names

  • Levothroid
  • Levoxyl
  • Unithroid
  • Synthroid
  • Tirosint
  • Levo-T

Note- This branded product is no longer on the market. Generic alternatives may be available.

Last Revised – 02/15/2019

Levothyroxine Drug Class

Levothyroxine is a synthetic thyroid hormone in the therapeutic and pharmacologic classes of hormones and thyroid preparations. Its primary function is to boost the metabolic rate of the body. This synthetic hormone is designed to mimic the effects of natural T4, produced by the thyroid gland.

Increased metabolic activity can improve energy levels, weight loss and promote overall physical health.

Levothyroxine is vital in effectively managing hypothyroidism, ensuring that individuals with this condition can lead healthy and active life. More specifically, the thyroid hormone has the following functions:

  • Plays a major role in brain and CNS development
  • Promotes gluconeogenesis and increased utilization of glycogen stores
  • Stimulates the synthesis of proteins
  • Encourages cells to grow and differentiate


Oral levothyroxine is FDA-approved for treating primary, secondary, and tertiary hypothyroidism;

Primary: most common cause is an autoimmune condition (Hashimoto thyroiditis) or iatrogenic hypothyroidism (following a thyroidectomy).

Secondary: hypothyroidism occurs when there is an issue with the pituitary gland (from pituitary adenomas to post-surgical intervention) with decreased thyroid stimulating hormone (TSH) production.

Tertiary: hypothyroidism is rare, and the problem lies in the hypothalamus, resulting in decreased thyroid-releasing hormone (TRH) production.

Levothyroxine is also approved for thyrotropin suppression to manage thyrotropin-dependent well-differentiated thyroid cancer. Injectable levothyroxine is used for treating myxedema coma or severe hypothyroidism. Additionally, off-label medication usage includes cadaveric organ recovery and subclinical hypothyroidism with TSH >10 mIU/L, and symptomatic patients are advised to undergo levothyroxine therapy.

Mechanism of Action

Levothyroxine (T4) is a man-made form of the thyroid hormone thyroxine (T4), which the body usually produces. The hypothalamus secretes Thyrotropin Releasing Hormone (TRH), which triggers the anterior pituitary to release Thyroid Stimulating Hormone (TSH). This stimulates the thyroid gland to secrete 80% T4 and 20% L-triiodothyronine (T3). The T4 is then converted into its active form, L-triiodothyronine (T3), which binds to particular proteins within cell nuclei. Thus, the thyroid hormones can exert their effects.

Once in the nucleus, thyroid hormones directly affect DNA transcription to boost metabolism by increasing gluconeogenesis, protein synthesis, and the mobilization of glycogen stores. When this process is disrupted (as seen with primary, secondary, or tertiary hypothyroidism), levothyroxine (LT4) can mimic the body’s natural T4 production by the thyroid.9,10. Thus, LT4 can help to restore normal metabolic processes and prevent associated symptoms of hypothyroidism.


Absorption: When taken orally, Levothyroxine is absorbed in the jejunum and upper ileum (40%-80%) at a bioavailability rate of 93% relative to an equal dose of oral levothyroxine solution. While fasting increases absorption, malabsorption syndromes and dietary fiber reduce its bioavailability.

Distribution: In the plasma, most thyroid hormones are bound to proteins like thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin. T4 has a greater affinity for TBG and TBPA; thus, it has a slower metabolic clearance and longer half-life than T3. Free hormone, which is metabolically active, can be altered by drugs or physiologic conditions.

Metabolism: Thyroid hormones are primarily degraded through sequential deiodination wherein 80% of T3 comes from peripheral T4. The liver is the primary site for T4 and T3 degradation; however, other areas like the kidney are also known to deiodinate T4. Subsequently, T3 and reverse T3 (rT3) are deiodinated into diiodothyronine. Thyroid hormones can also be metabolized through conjugation and undergo enterohepatic recirculation.

Excretion: The kidneys mainly excrete thyroid hormones, while 20% of T4 is excreted in the stool. The urinary excretion of T4 diminishes with age.

Route of Elimination

The kidneys mainly excrete thyroid hormones. A part of the conjugated hormone remains intact and is eliminated in the feces. Approximately 20% of T4 is excreted via stool. As a person ages, their urinary elimination of T4 decreases.

Levothyroxine PT Teaching (Administration)

Different dosage forms are available when taking levothyroxine, such as oral tablets, capsules, solutions, and parenteral medications.

The following instructions should be taken into account when using the different types of dosage forms:

Oral: When taking it orally, the tablet or capsule should be consumed on an empty stomach at least 30 to 60 minutes before breakfast or 3 to 4 hours after dinner. Additionally, avoid taking levothyroxine within four hours of any iron or calcium product and when taking antacids or proton pump inhibitors.

Capsule: When taking a capsule, swallow it whole and avoid crushing or cutting it.

Tablet: To prevent dysphagia, the tablet should be swallowed with a full glass of water or crushed into 5 to 10 mL before drinking it immediately.

Solution: When taking the solution, either consume it undiluted (by squeezing it directly into the mouth) or dilute it in water before consuming.

Finally, intravenous levothyroxine is only used in hospitals where vital signs can be closely monitored. In cases where patients cannot take anything orally, levothyroxine capsules are used as a suppository and are well absorbed.

Normal Dose Range

The dose of levothyroxine should be individualized based on patient response and laboratory values. The starting amount for pediatric patients is usually around 12.5 to 25 mcg/day, which can be increased per clinical response. In adults, the initial dose is usually about 50 to 100 mcg/day, which can be increased at intervals of 4 to 6 weeks. The usual maintenance dose in adults is around 1.6 to 1.7 mcg/kg/day or 100 to 200 mcg/day, depending on the patient’s clinical response and metabolism levels.

The dose should be decreased when thyrotropin levels are less than 0.4 mU/L and increased when thyrotropin levels are higher than 4.5 mU/L. The dosage may be titrated if necessary until the desired response is achieved. The dose should also be adjusted for the elderly and patients with renal failure.

Side Effects

Common side effects of levothyroxine include:

  • Headache
  • Insomnia
  • Nervousness
  • Sweating
  • Fatigue.
  • Muscle cramps
  • Shaking, usually of the hands
  • Feeling restless or excitable or having problems sleeping

Additionally, some patients taking levothyroxine may have more severe side effects, such as chest pain, rapid or irregular heartbeats, shortness of breath, and muscle weakness. If these symptoms occur, the patient should seek immediate medical attention.

Drug Interactions

Levothyroxine may interact with certain drugs to treat depression, diabetes, and heart problems. It can also interact with birth control pills, cold medications, and other over-the-counter medications. Therefore, the patient must inform their doctor or pharmacist of any medications they are taking before starting levothyroxine therapy.


Patients receiving levothyroxine should be monitored for clinical response and serum thyrotropin levels. Additionally, laboratory tests such as total T3, free T4, reverse T3, and thyroid autoantibodies can also be used to assess the effectiveness of therapy. Furthermore, monitoring vital signs such as heart rate, blood pressure, and respiratory rate is also essential.

Guidelines for Administration

Safety: Levothyroxine should be used cautiously in patients with cardiovascular disease, hypertension, diabetes mellitus, and thyroid disorders. It is also not recommended for pregnant women, as it can cause fetal harm.

Storage: Levothyroxine should be stored at room temperature and protected from light. It should be kept away from children and pets.

Disposal: Any unused levothyroxine should be disposed of by local regulations. Patients should consult their healthcare provider for specific instructions on removal of the medication.

Levothyroxine has many drug-drug interactions. Some notable ones are:

  1. Phenytoin: Increases the metabolism of levothyroxine, leading to a decrease in its efficacy.
  2. Estrogens: This may cause increased levothyroxine metabolism and therefore reduce its efficacy.
  3. Tricyclic antidepressants (TCA): May interfere with the absorption of levothyroxine, leading to decreased efficacy.
  4. Carbamazepine: This may increase the metabolism of levothyroxine, resulting in a decrease in its effectiveness.
  5. Calcium and iron products: They should not be taken within four hours of levothyroxine as they can reduce its absorption and efficacy.

It is essential to be aware of drug interactions with levothyroxine and discuss them with a healthcare provider. Also, patients should inform their healthcare provider if they are taking any of these medications before starting levothyroxine therapy.

Patients should also follow the guidelines for administration, storage, and disposal. Finally, patients should be aware of the side effects and potential drug interactions with levothyroxine and consult a healthcare provider if any adverse effects occur. With proper use and monitoring, levothyroxine is an effective treatment for hypothyroidism.

Levothyroxine PT Teaching: Nursing Implications

It is essential to take levothyroxine as instructed by your doctor or pharmacist.

  • Oral: Take the tablet/capsule on an empty stomach at least 30-60 minutes before breakfast or 3-4 hours after dinner. Do not take levothyroxine within four hours of taking iron/calcium supplements.
  • Injection: Your doctor or pharmacist will instruct you on taking your medication best.

It is essential to keep taking levothyroxine even when you feel well, as it takes several weeks for the full therapeutic effects of the medication to be noticed. Make regular appointments with your doctor to review your thyroid levels and adjust the levothyroxine dosage if needed.

Also, be sure to inform any healthcare professionals, including dentists, that you are taking levothyroxine before undergoing any treatments or surgery. Lastly, never stop taking this medication without talking to your doctor first. Doing so could cause serious health complications.

If you have any questions about your levothyroxine therapy, consult your doctor or pharmacist to ensure you understand the instructions for taking this medication.

Final Words

Levothyroxine is a widely used drug for treating hypothyroidism. Depending on the patient’s needs and response, it can be taken orally or intravenously. Patients must inform their doctor of any medications they are taking before starting levothyroxine therapy, as it may interact with certain drugs.

Levothyroxine class should be monitored for clinical response and serum thyrotropin levels. Additionally, laboratory tests such as total T3, free T4, reverse T3, and thyroid autoantibodies can also be used to assess the effectiveness of therapy. Furthermore, monitoring vital signs such as heart rate, blood pressure, and respiratory rate is also essential. Patients must follow the guidelines for administration, storage, and disposal of levothyroxine as outlined by their healthcare provider. With proper use and monitoring, levothyroxine is an effective treatment for hypothyroidism.





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