Levothyroxine; Uses, Dosage, Side Effects, Interactions

Last updated: February 7, 2026Reviewed date: February 7, 2026Reading time: 9 min read
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Levothyroxine is a synthetic levoisomer of thyroxine (T4), similar to the endogenous hormone produced by the thyroid gland. Thyroxine is de-iodinated to form triiodothyronine (T3) in the peripheral tissues. T3 enters the cell and binds to nuclear thyroid hormone receptors, and the hormone-receptor complex, in turn, triggers gene expression and produces proteins...

For severe symptoms, danger signs, pregnancy, child illness, or sudden worsening, seek urgent medical care.

বাংলা রোগী নোট এখনো যোগ করা হয়নি। পোস্ট এডিটরে “RX Bangla Patient Mode” বক্স থেকে সহজ বাংলা সারাংশ যোগ করুন।

এই তথ্য শিক্ষা ও সচেতনতার জন্য। এটি ডাক্তারি পরীক্ষা, রোগ নির্ণয় বা প্রেসক্রিপশনের বিকল্প নয়।

Article Summary

Levothyroxine is a synthetic levoisomer of thyroxine (T4), similar to the endogenous hormone produced by the thyroid gland. Thyroxine is de-iodinated to form triiodothyronine (T3) in the peripheral tissues. T3 enters the cell and binds to nuclear thyroid hormone receptors, and the hormone-receptor complex, in turn, triggers gene expression and produces proteins required in the regulation of cellular respiration, thermogenesis, cellular growth and differentiation, and metabolism of proteins, carbohydrates, and lipids. T4...

Key Takeaways

  • This article explains Mechanism of Action of Levothyroxine in simple medical language.
  • This article explains Indications of Levothyroxine in simple medical language.
  • This article explains Contra-Indications of Levothyroxine in simple medical language.
  • This article explains Dosage of Levothyroxine in simple medical language.
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Definition

Levothyroxine is a synthetic levoisomer of thyroxine (T4), similar to the endogenous hormone produced by the thyroid gland. Thyroxine is de-iodinated to form triiodothyronine (T3) in the peripheral tissues. T3 enters the cell and binds to nuclear thyroid hormone receptors, and the hormone-receptor complex, in turn, triggers gene expression and produces proteins required in the regulation of cellular respiration, thermogenesis, cellular growth and differentiation, and metabolism of proteins, carbohydrates, and lipids. T4 and T3 also possess cardiac stimulatory effect.

Levothyroxine is a manufactured form of the thyroid hormone, thyroxine (T4). It is used to treat thyroid hormone deficiency including the severe form known as myxedema coma. It may also be used to treat and prevent certain types of thyroid tumors. It is not indicated for weight loss. Levothyroxine is taken by mouth or given by injection into a vein. Maximum effect from a specific dose can take up to six weeks to occur. The major hormone derived from the thyroid gland.

Mechanism of Action of Levothyroxine

Levothyroxine acts like the endogenous thyroid hormone thyroxine (T4, a tetra-iodinated tyrosine derivative). In the liver and kidney, T4 is converted to T3, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxine-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then take place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxine deficiency are relieved. Levothyroxine (T4) is a synthetically prepared levo isomer of thyroxine, the major hormone secreted from the thyroid gland. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine (T3) which exerts a broad spectrum of stimulatory effects on cell metabolism. Thyroid hormone increases the metabolic rate of cells of all tissues in the body.

or

Levothyroxine acts like the endogenous thyroid hormone thyroxine (T4, a tetra-iodinated tyrosine derivative). In the liver and kidney, T4 is converted to T3, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxine-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then take place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxin deficiency are relieved.

Indications of Levothyroxine

  • thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।" data-rx-term="hypothyroidism" data-rx-definition="Hypothyroidism means the thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।">Hypothyroidism
  • Hashimoto’s disease
  • Underactive thyroid
  • thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।" data-rx-term="hypothyroidism" data-rx-definition="Hypothyroidism means the thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।">Hypothyroidism, after thyroid removal
  • TSH suppression
  • Thyroid suppression test
  • Myxedema Coma
  • Euthyroid goiter
  • Thyrotropin dependent thyroid cancer
  • For use alone or in combination with antithyroid agents to treat thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।" data-rx-term="hypothyroidism" data-rx-definition="Hypothyroidism means the thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।">hypothyroidism, goiter, chronic lymphocytic thyroiditis, myxedema coma, and stupor.

Contra-Indications of Levothyroxine

  • Overactive thyroid gland
  • Thyrotoxicosis crisis
  • insulin is low or not working well. সহজ বাংলা: রক্তে চিনি বেশি থাকার রোগ।" data-rx-term="diabetes" data-rx-definition="Diabetes is a condition where blood sugar stays too high because insulin is low or not working well. সহজ বাংলা: রক্তে চিনি বেশি থাকার রোগ।">Diabetes
  • Pituitary hormone deficiency
  • Addison’s disease
  • fracture risk. সহজ বাংলা: হাড় দুর্বল হয়ে ভাঙার ঝুঁকি বেশি।" data-rx-term="osteoporosis" data-rx-definition="Osteoporosis means weak, fragile bones with higher fracture risk. সহজ বাংলা: হাড় দুর্বল হয়ে ভাঙার ঝুঁকি বেশি।">Osteoporosis
  • Decreased calcification or density of bone
  • Allergies to thyroid

Dosage of Levothyroxine

Strengths: 25 mcg; 50 mcg;75 mcg ;100 mcg;125 mcg ;150 mcg ;200 mcg ; 300 mcg

thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।" data-rx-term="hypothyroidism" data-rx-definition="Hypothyroidism means the thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।">Hypothyroidism

Elderly patients with cardiac disease 

  • Initial dose: 12.5 to 25 mcg orally per day, with gradual dose increments at 4 to 6-week intervals
  • The dose is generally adjusted in 12.5 to 25 mcg increments until the patient with primary thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।" data-rx-term="hypothyroidism" data-rx-definition="Hypothyroidism means the thyroid gland makes too little hormone. সহজ বাংলা: থাইরয়েড হরমোন কম।">hypothyroidism is clinically euthyroid and the serum TSH has normalized.

TSH Suppression

TSH suppression in well-differentiated thyroid cancer &thyroid nodules

  • The target level for TSH suppression in these conditions has not been established. The efficacy of TSH suppression for the benign nodular disease is controversial. Therefore, the dose of this drug used for TSH suppression should be individualized based on the specific disease and the patient being treated.
  • In the treatment of well differentiated (papillary and follicular) thyroid cancer, this drug is used as an adjunct to surgery and radioiodine therapy. Generally, TSH is suppressed to less than 0.1 international units per liter (mg/L), and this usually requires a dose of greater than 2 mcg/kg/day. However, in patients with high-risk tumors, the target level for TSH suppression may be less than 0.01 mU/L.
  • In the treatment of benign nodules and nontoxic multinodular goiter, TSH is generally suppressed to a higher target (e.g., 0.1 to 0.5 mU/L for nodules and 0.5 to 1.0 mU/L for multinodular goiter) than that used for the treatment of thyroid cancer. This drug is contraindicated if the serum TSH is already suppressed due to the risk of precipitating overt thyrotoxicosis.

Hypothyroidism

Hypothyroidism  in adult and in children in whom growth& puberty and complete

  • Average full replacement dose: 1.7 mcg/kg/day (e.g., 100 to 125 mcg/day for a 70 kg adult) orally
  • Older patients may require less than 1 mcg/kg/day
  • Doses greater than 200 mcg/day orally are seldom required
  • An inadequate response to oral daily doses of 300 mcg/day or greater is rare and may indicate poor compliance, malabsorption, and/or drug interactions

For most patients older than 50 years or for patients under 50years of age with the underlying cardiac disease

  • Initial dose: 25 to 50 mcg/day, with gradual increments in dose at 6 to 8-week intervals, as needed

Patients with  hyperthyroidism

  • Initial dose: 12.5 to 50 mcg orally once a day; dosage can be increased in 12.5 to 25 mcg/day increments every 2 to 4 weeks (accompanied by clinical and laboratory assessment, until the TSH level is normalized)

Patients with secondary (pituitary) or tertiary(hypothalamic) hyperthyroidism

  • The oral dose should be titrated until the patient is clinically euthyroid and the serum free-T4 level is restored to the upper half of the normal range

Side Effects of Levothyroxine

The most common

More common

Less common

  • change in sense of taste
  • congestion
  • discouragement, feeling sad, or empty
  • Acting on dangerous impulses
  • New or worse depression
  • New or worse anxiety or panic attacks
  • Agitation, restlessness, anger, or irritability
  • Trouble sleeping
  • An increase in activity or talking more than normal

Drug Interactions of Levothyroxine

Levothyroxine may interact with following drugs, supplements& may decrease the efficacy of the drugs

Pregnancy & Lactation 

FDA Pregnancy Category A 

Pregnancy

Levothyroxine should be taken throughout pregnancy to  regulate the levels of thyroid hormone for the pregnant mother and the developing baby. If you become pregnant while taking this medication, contact your doctor as soon as possible. Your doctor may want to monitor your thyroid function more closely while you are pregnant.

Breast-feeding

Only a small amount of thyroid hormone is passed into breast milk. The use of appropriate amounts of this medication by breast-feeding women has not been shown to cause harm for breast-fed babies.

References

Levothyroxine; Uses, Dosage, Side Effects, Interactions

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Get urgent help if

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Doctor to discuss: Orthopedic / spine specialist, physical medicine doctor, or qualified clinician
Tests to discuss with doctor
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Questions to ask
  • What is the most likely cause of my symptoms?
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Care roadmap for: Levothyroxine; Uses, Dosage, Side Effects, Interactions

Use this simple roadmap to understand the next safe steps. It is educational and does not replace examination by a doctor.

Go to emergency care if you notice:
  • Severe or rapidly worsening symptoms
  • Breathing difficulty, chest pain, fainting, confusion, severe weakness, major injury, or severe dehydration
Doctor / service to discuss: Qualified healthcare provider; specialist depends on symptoms and examination.
  1. Step 1

    Check danger signs first

    If danger signs are present, seek emergency care and do not wait for online information.

  2. Step 2

    Record the symptom story

    Write when symptoms started, severity, medicines already taken, allergies, pregnancy status, and test results.

  3. Step 3

    Visit a qualified clinician

    A doctor, nurse, or qualified healthcare provider can examine you and decide which tests or treatment are needed.

  4. Step 4

    Do only useful tests

    Do tests after clinical assessment. Avoid unnecessary tests, random antibiotics, or repeated medicines without diagnosis.

  5. Step 5

    Follow up and return early if worse

    If symptoms worsen, new warning signs appear, or treatment is not helping, return for review quickly.

Rural patient practical tips
  • Take a written symptom diary and all previous prescriptions/test reports.
  • Do not hide medicines already taken, even herbal or over-the-counter medicines.
  • Ask which warning signs mean urgent referral to hospital.

This roadmap is for education. A real diagnosis and treatment plan requires history, examination, and clinical judgment.

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