Medithyrox: Uses, Dosage & Side Effects
A synthetic thyroid hormone (levothyroxine) used for replacement therapy in hypothyroidism and TSH suppression in thyroid cancer management
Medithyrox is a brand of levothyroxine sodium, the standard treatment for hypothyroidism (underactive thyroid gland). Levothyroxine is a synthetic form of thyroxine (T4), the principal hormone produced by the thyroid gland. It is listed on the World Health Organization's List of Essential Medicines and is one of the most prescribed medications globally, with over 100 million prescriptions dispensed annually in the United States alone. Medithyrox is taken once daily on an empty stomach to restore normal thyroid hormone levels, alleviating symptoms such as fatigue, weight gain, cold intolerance, and cognitive impairment. Accurate dosing is critical, as even small variations in levothyroxine bioavailability can significantly affect clinical outcomes.
Quick Facts: Medithyrox
Key Takeaways
- Medithyrox (levothyroxine) is the first-line treatment for hypothyroidism, replacing the thyroid hormone your body cannot produce in sufficient amounts.
- Take it once daily on an empty stomach, 30-60 minutes before breakfast, with a full glass of water for optimal absorption.
- Dose adjustments are based on TSH blood tests, typically checked every 6-8 weeks until stable levels are achieved.
- Avoid taking calcium, iron, or antacid supplements within 4 hours of Medithyrox, as they significantly reduce absorption.
- Medithyrox is safe and essential during pregnancy; hypothyroid women often require a 25-50% dose increase in early pregnancy.
What Is Medithyrox and What Is It Used For?
Medithyrox is a pharmaceutical preparation of levothyroxine sodium, which is chemically identical to thyroxine (T4), the primary hormone secreted by the thyroid gland. The thyroid gland, a butterfly-shaped organ located at the base of the neck, plays a fundamental role in regulating the body's metabolism, energy production, growth, and development. When the thyroid gland fails to produce sufficient hormones — a condition known as hypothyroidism — virtually every organ system in the body can be affected.
Hypothyroidism is remarkably common, affecting approximately 5% of the general population worldwide, with a higher prevalence among women and older adults. The most frequent cause in iodine-sufficient regions is Hashimoto's thyroiditis, an autoimmune condition in which the body's immune system gradually destroys thyroid tissue. Other causes include surgical removal of the thyroid (thyroidectomy), radioactive iodine treatment for hyperthyroidism or thyroid cancer, certain medications (such as amiodarone and lithium), and congenital hypothyroidism in newborns.
Levothyroxine replacement therapy with Medithyrox works by supplementing or replacing the body's natural T4 production. After oral ingestion, levothyroxine enters the bloodstream and is distributed to tissues throughout the body. In peripheral tissues, particularly the liver and kidneys, T4 is converted to triiodothyronine (T3) by deiodinase enzymes. T3 is the biologically active form of thyroid hormone that binds to nuclear receptors and regulates gene expression controlling metabolism, protein synthesis, thermogenesis, and organ function.
The primary indications for Medithyrox include:
- Primary hypothyroidism: Replacement therapy for inadequate endogenous thyroid hormone production, regardless of cause (autoimmune, post-surgical, post-radiation, congenital, or drug-induced).
- TSH suppression: In patients with well-differentiated thyroid cancer who have undergone thyroidectomy, levothyroxine is used at supraphysiological doses to suppress TSH and reduce the risk of cancer recurrence.
- Myxedema coma: In emergency settings, intravenous levothyroxine is used for severe, life-threatening hypothyroidism (though Medithyrox tablets are for chronic oral therapy).
- Goiter: Levothyroxine may be used to suppress TSH-driven growth of non-toxic goiter in selected patients.
- Diagnostic use: In thyroid suppression tests to differentiate suspected mild hyperthyroidism from thyroid gland autonomy.
Levothyroxine has been in clinical use for over 60 years and is one of the most extensively studied medications in history. The American Thyroid Association (ATA), European Thyroid Association (ETA), and other international endocrine societies universally recommend levothyroxine monotherapy as the standard of care for hypothyroidism. It is listed on the WHO Model List of Essential Medicines, recognizing its fundamental importance in global healthcare.
What Should You Know Before Taking Medithyrox?
While levothyroxine is generally considered a very safe medication when dosed appropriately, there are several important considerations and precautions that must be addressed before initiating therapy. A thorough medical history and baseline laboratory evaluation are essential for safe and effective treatment.
Contraindications
Medithyrox should not be used in the following situations:
- Untreated adrenal insufficiency: Starting levothyroxine before correcting adrenal insufficiency (Addison's disease or secondary adrenal insufficiency) can precipitate an adrenal crisis, a potentially life-threatening emergency. Cortisol replacement must be initiated before or concurrently with thyroid hormone therapy.
- Acute myocardial infarction: Levothyroxine should not be started during an acute heart attack unless there is severe underlying hypothyroidism contributing to the cardiac event.
- Untreated thyrotoxicosis: Adding exogenous thyroid hormone when endogenous levels are already elevated can cause dangerous hyperthyroid symptoms.
- Hypersensitivity: Allergy to levothyroxine sodium or any excipient in the formulation. True levothyroxine allergy is exceedingly rare; reactions are more commonly caused by inactive ingredients (dyes, fillers).
Thyroid hormones, including levothyroxine, should not be used for the treatment of obesity or weight loss. In euthyroid patients, doses within the range of normal hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when combined with sympathomimetic amines such as those used for appetite suppression.
Warnings and Precautions
Special caution is required in the following patient groups:
- Cardiovascular disease: Patients with coronary artery disease, heart failure, or cardiac arrhythmias should start levothyroxine at a low dose (12.5-25 mcg daily) with gradual increases every 4-6 weeks. Thyroid hormone increases cardiac oxygen demand and can exacerbate angina, arrhythmias, or trigger acute coronary events in susceptible individuals.
- Elderly patients: Age-related decline in cardiac reserve and altered drug metabolism require lower starting doses and slower titration. Starting doses of 12.5-25 mcg are recommended in patients over 50 years with known or suspected heart disease.
- Diabetes mellitus: Initiating thyroid hormone therapy may increase blood glucose levels and require adjustment of insulin or oral hypoglycemic medications. Close blood glucose monitoring is recommended during the initial dosing phase.
- Osteoporosis: Long-term supraphysiologic doses of levothyroxine (as used in TSH suppression therapy) may accelerate bone loss, particularly in postmenopausal women. Bone density monitoring is recommended.
- Seizure disorders: Rarely, initiating levothyroxine therapy can lower the seizure threshold. Patients with epilepsy should be monitored closely during dose initiation.
Pregnancy and Breastfeeding
Levothyroxine is classified as safe for use during pregnancy and breastfeeding. In fact, maintaining adequate thyroid hormone levels is essential for healthy pregnancy outcomes and fetal neurodevelopment. Untreated maternal hypothyroidism during pregnancy is associated with increased risks of:
- Miscarriage and stillbirth
- Preeclampsia and gestational hypertension
- Placental abruption
- Preterm delivery
- Low birth weight
- Impaired cognitive and neurological development in the child
During pregnancy, the body's demand for thyroid hormone increases significantly, particularly during the first trimester when the fetus depends entirely on maternal thyroid hormone supply. Women with hypothyroidism who become pregnant typically require a dose increase of 25-50% within the first 4-6 weeks of pregnancy. Current guidelines from the ATA recommend that TSH should ideally be maintained below 2.5 mIU/L during the first trimester. Regular TSH monitoring every 4 weeks during the first half of pregnancy and at least once during the second half is recommended.
Levothyroxine is excreted in breast milk in minimal amounts. These amounts are insufficient to cause any thyroid effects in the nursing infant, and breastfeeding is not only safe but encouraged for women taking levothyroxine. The postpartum dose should be reduced back to the pre-pregnancy level after delivery, with TSH levels rechecked 6-8 weeks postpartum.
How Does Medithyrox Interact with Other Drugs?
Levothyroxine has a narrow therapeutic index, meaning small changes in the amount of drug absorbed or metabolized can significantly affect clinical outcomes. Drug interactions with levothyroxine fall into three main categories: agents that reduce gastrointestinal absorption, agents that alter thyroid hormone binding to plasma proteins, and agents that affect thyroid hormone metabolism.
Understanding these interactions is crucial for maintaining stable thyroid hormone levels and avoiding both under- and over-treatment. Patients taking levothyroxine should always inform their healthcare provider and pharmacist about all medications, supplements, and herbal products they are using.
Major Interactions
| Interacting Drug/Substance | Mechanism | Clinical Effect | Management |
|---|---|---|---|
| Calcium supplements | Chelation in GI tract reduces absorption | Up to 40% reduction in levothyroxine absorption | Separate by at least 4 hours |
| Iron supplements | Forms insoluble complex in stomach | Significant reduction in T4 absorption | Separate by at least 4 hours |
| Proton pump inhibitors (omeprazole, pantoprazole) | Reduced gastric acidity impairs dissolution | Reduced absorption; may require dose increase | Monitor TSH; consider dose adjustment |
| Cholestyramine / Colestipol | Bile acid sequestrants bind levothyroxine in GI tract | Markedly reduced absorption | Separate by at least 4-6 hours |
| Warfarin | Levothyroxine enhances catabolism of vitamin K-dependent clotting factors | Increased anticoagulant effect; bleeding risk | Monitor INR closely when initiating or adjusting levothyroxine dose |
| Carbamazepine / Phenytoin | Induce hepatic metabolism of T4 and displace from protein binding | Reduced serum T4 levels; potential hypothyroidism | Monitor thyroid function; may need dose increase |
| Rifampicin | Potent CYP enzyme inducer increases T4 clearance | Reduced levothyroxine efficacy | Monitor TSH; increase levothyroxine dose as needed |
Minor Interactions
Several other substances can have lesser but clinically relevant effects on levothyroxine therapy:
- Aluminum-containing antacids: Can reduce absorption; separate by 4 hours.
- Soy products: Dietary soy may reduce absorption of levothyroxine. Patients consuming large amounts of soy should have their TSH monitored and dose adjusted if necessary.
- Coffee: Espresso and coffee can reduce the absorption of levothyroxine if taken simultaneously. Wait at least 30 minutes after taking levothyroxine before drinking coffee.
- Fiber supplements: High-fiber diets and supplements can reduce absorption. Maintain a consistent fiber intake rather than making sudden changes.
- Estrogens (oral contraceptives, HRT): Estrogen increases thyroxine-binding globulin (TBG), which may increase the levothyroxine dose requirement in hypothyroid women.
- Biotin (vitamin B7): High-dose biotin does not interact with levothyroxine itself but can interfere with laboratory thyroid function tests, producing falsely abnormal TSH and T4 results. Discontinue biotin at least 48 hours before thyroid blood tests.
- Sucralfate: Binds to levothyroxine in the stomach, reducing absorption. Separate by at least 8 hours.
To minimize interaction risks, take Medithyrox first thing in the morning on an empty stomach with plain water. Wait 30-60 minutes before eating, drinking coffee, or taking other medications. Take calcium, iron, and antacid supplements at lunch or in the evening, at least 4 hours after your levothyroxine dose.
What Is the Correct Dosage of Medithyrox?
Levothyroxine dosing is highly individualized and depends on the patient's age, weight, the severity of hypothyroidism, the underlying cause, the presence of cardiovascular disease, and the specific treatment goal (replacement vs. TSH suppression). The goal of replacement therapy in hypothyroidism is to normalize TSH levels, typically within the reference range of 0.4-4.0 mIU/L, though many clinicians aim for the lower half of this range (0.4-2.5 mIU/L), particularly in younger patients.
Adults
Standard Adult Dosing
The full replacement dose for most adults is approximately 1.6 mcg/kg/day based on ideal body weight. For a 70 kg adult, this translates to approximately 112 mcg daily. In practice, most adults are stabilized on doses between 75-200 mcg daily.
- Young, healthy adults: Can often be started on the full replacement dose or near-full dose (e.g., 50-100 mcg daily).
- Partial thyroidectomy or mild hypothyroidism: Lower initial doses (25-50 mcg) may be sufficient.
- TSH suppression (thyroid cancer): Higher doses (150-300 mcg) targeting TSH below 0.1-0.5 mIU/L depending on risk category.
| Patient Group | Starting Dose | Titration | Target TSH |
|---|---|---|---|
| Young, healthy adults | 50-100 mcg/day | Increase by 25-50 mcg every 6-8 weeks | 0.4-2.5 mIU/L |
| Elderly (>65 years) | 12.5-25 mcg/day | Increase by 12.5-25 mcg every 6-8 weeks | 0.4-6.0 mIU/L (age-adjusted) |
| Cardiac disease | 12.5-25 mcg/day | Increase by 12.5-25 mcg every 4-8 weeks | Individualized; avoid suppression |
| Pregnancy | Increase pre-pregnancy dose by 25-50% | Adjust per TSH every 4 weeks | <2.5 mIU/L (1st trimester) |
| Thyroid cancer (TSH suppression) | Higher than replacement; individualized | Guided by TSH and cancer risk category | <0.1-0.5 mIU/L (risk-dependent) |
Children
Thyroid hormone is essential for normal growth and neurodevelopment in children. Congenital hypothyroidism, if left untreated, can lead to irreversible intellectual disability. Newborn screening programs detect congenital hypothyroidism at birth, enabling early treatment initiation.
Pediatric Dosing
- Neonates (0-3 months): 10-15 mcg/kg/day. Prompt initiation is critical for neurodevelopment.
- Infants (3-6 months): 8-10 mcg/kg/day.
- Infants (6-12 months): 6-8 mcg/kg/day.
- Children (1-5 years): 5-6 mcg/kg/day.
- Children (6-12 years): 4-5 mcg/kg/day.
- Adolescents (12-18 years): 2-3 mcg/kg/day, approaching adult dosing.
Pediatric doses are higher per kilogram than adult doses because children have a higher metabolic rate and faster thyroid hormone clearance. Growth and development should be monitored alongside TSH and free T4 levels.
Elderly
Elderly patients require particular caution when initiating levothyroxine therapy. Age-related changes in cardiovascular function, reduced renal clearance, and altered drug metabolism all contribute to increased sensitivity to thyroid hormone. The starting dose should be low (12.5-25 mcg daily), with careful upward titration at intervals of 6-8 weeks. In patients over 70-80 years, some guidelines suggest higher TSH targets (up to 6-7 mIU/L) may be appropriate, as subclinical hypothyroidism in the very elderly is not consistently associated with adverse outcomes and aggressive treatment may carry cardiovascular risk.
Missed Dose
If you miss a dose of Medithyrox, take it as soon as you remember on the same day. If you do not remember until the next day, skip the missed dose and resume your regular schedule. Do not take a double dose. Because levothyroxine has a long half-life of approximately 6-7 days, missing a single dose is unlikely to cause noticeable symptoms or significant changes in thyroid hormone levels. However, consistent daily dosing is important for maintaining stable levels over time. Some physicians may recommend taking two pills the next day if a dose is missed, but this approach should only be followed on the advice of your doctor.
Overdose
Levothyroxine overdose can produce signs and symptoms of hyperthyroidism: tachycardia, tremor, anxiety, insomnia, diarrhea, excessive sweating, fever, and in severe cases, seizures, cardiac arrhythmias, or heart failure. Acute massive overdose is a medical emergency. Seek immediate medical attention if a large amount has been ingested. Treatment may include beta-blockers, activated charcoal (if within 1-2 hours), and supportive care. Symptoms may be delayed by several days due to the slow onset of thyroid hormone action.
What Are the Side Effects of Medithyrox?
It is important to distinguish between true side effects of levothyroxine and symptoms caused by incorrect dosing. Levothyroxine is a bioidentical replacement for a hormone your body naturally produces. When the dose is correct and thyroid levels are within the normal range, the medication should cause no adverse effects. The vast majority of reported side effects are dose-dependent and result from either overtreatment (excessive dose leading to hyperthyroid symptoms) or undertreatment (insufficient dose leaving hypothyroid symptoms unresolved).
In clinical practice, symptoms of overtreatment are the most commonly encountered issues. These typically emerge gradually as the dose accumulates and can usually be resolved by reducing the levothyroxine dose. The following frequency classification is based on data from clinical trials and post-marketing surveillance.
Common Side Effects
Affects 1-10 in 100 people (typically from dose adjustment)
- Palpitations or rapid heartbeat (tachycardia)
- Tremor (fine hand tremor)
- Restlessness, nervousness, or anxiety
- Headache
- Insomnia or difficulty sleeping
- Increased sweating or heat intolerance
- Diarrhea or increased bowel frequency
- Transient hair loss (usually within first few months of treatment; self-limiting)
Uncommon Side Effects
Affects 1-10 in 1,000 people
- Muscle cramps or weakness
- Menstrual irregularities
- Flushing or feeling of warmth
- Appetite changes (increased appetite)
- Vomiting or abdominal discomfort
- Weight loss (unintentional, from over-replacement)
Rare Side Effects
Affects fewer than 1 in 1,000 people
- Cardiac arrhythmias (atrial fibrillation)
- Angina pectoris (chest pain) — primarily in patients with pre-existing coronary artery disease
- Seizures (extremely rare, primarily at treatment initiation)
- Allergic reactions to excipients (rash, urticaria)
- Pseudotumor cerebri (benign intracranial hypertension) in children
- Decreased bone mineral density (with long-term supraphysiological dosing)
Not Known (Frequency Cannot Be Estimated)
Isolated reports from post-marketing surveillance
- Craniosynostosis (premature closure of skull sutures) in infants on prolonged overtreatment
- Slipped capital femoral epiphysis in adolescents on high doses
Contact your doctor promptly if you experience chest pain, rapid or irregular heartbeat, shortness of breath, severe headache, signs of allergic reaction (rash, swelling, difficulty breathing), or if you notice any persistent symptom that could indicate your dose needs adjustment. Regular thyroid function monitoring is the best way to prevent dose-related side effects.
Long-term use of levothyroxine at appropriate replacement doses has an excellent safety profile. Large observational studies involving hundreds of thousands of patients have confirmed that levothyroxine replacement therapy, when properly monitored, does not increase the risk of cardiovascular events, fractures, or mortality compared to the general population. The key to safety is regular monitoring and dose adjustment to maintain TSH within the target range.
How Should You Store Medithyrox?
Proper storage of Medithyrox is important to ensure the medication maintains its potency and effectiveness throughout its shelf life. Levothyroxine is a relatively stable molecule, but it can degrade when exposed to excessive heat, moisture, or light, which could reduce the amount of active drug in each tablet and lead to undertreatment.
Follow these storage guidelines:
- Temperature: Store at room temperature between 15°C and 25°C (59°F to 77°F). Brief excursions up to 30°C (86°F) are generally acceptable, but prolonged exposure to high temperatures should be avoided.
- Light: Keep the tablets in their original blister packaging or container to protect them from light. Do not transfer tablets to a clear container without light protection.
- Moisture: Store in a dry place. Avoid storing in the bathroom, where humidity levels are typically high. Do not remove tablets from blister packs until ready to take them.
- Children: Keep all medications out of the sight and reach of children. Accidental ingestion of levothyroxine by children can cause symptoms of hyperthyroidism and requires medical attention.
- Disposal: Do not flush unused tablets down the toilet or drain. Return expired or unused medication to your pharmacy for safe disposal according to local regulations.
Check the expiration date on the packaging regularly. Do not use Medithyrox after the stated expiration date. If you notice any change in the appearance of the tablets (discoloration, crumbling, unusual smell), do not take them and consult your pharmacist for replacement.
What Does Medithyrox Contain?
Each Medithyrox tablet contains levothyroxine sodium as the active pharmaceutical ingredient. Levothyroxine sodium is the sodium salt of the naturally occurring L-isomer of thyroxine (T4), the principal hormone secreted by the thyroid gland. The chemical formula is C15H10I4NNaO4, and its molecular weight is approximately 798.86 g/mol.
The wide range of available strengths (13, 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, and 200 micrograms) allows for precise dose titration. The 13 mcg tablet is particularly useful for very fine dose adjustments and for elderly patients or those with cardiovascular disease who require very low starting doses.
Typical inactive ingredients (excipients):
- Mannitol: A sugar alcohol used as a filler and to improve tablet stability.
- Corn starch: Functions as a binder and disintegrant.
- Gelatin: Used as a binding agent to hold the tablet together.
- Croscarmellose sodium: A superdisintegrant that promotes rapid tablet disintegration in the stomach.
- Magnesium stearate: A lubricant used in tablet manufacturing to prevent ingredients from sticking to machinery.
- Citric acid (anhydrous): Helps maintain tablet stability and pH.
Different brands and generic versions of levothyroxine may contain different excipients (inactive ingredients). Because levothyroxine has a narrow therapeutic index, switching between different brands or manufacturers can sometimes cause small but clinically significant changes in absorption and bioavailability. If you switch brands or formulations, your doctor may recommend rechecking your TSH levels after 6-8 weeks. Patients who experience problems with one formulation may benefit from trying another due to differences in excipients.
Frequently Asked Questions About Medithyrox
Medithyrox contains levothyroxine sodium, a synthetic thyroid hormone used to treat hypothyroidism (underactive thyroid). It replaces or supplements the thyroid hormone your body cannot produce in sufficient quantities. Medithyrox is also used for thyroid-stimulating hormone (TSH) suppression in the management of thyroid cancer and as a diagnostic agent in thyroid suppression tests. It is one of the most commonly prescribed medications worldwide, listed on the WHO's List of Essential Medicines.
Take Medithyrox once daily on an empty stomach, ideally 30 to 60 minutes before breakfast. Swallow the tablet whole with a full glass of water. Consistency is key — take it at the same time every day. Avoid taking calcium, iron supplements, or antacids within 4 hours, as they significantly reduce absorption. Coffee should also be avoided for at least 30 minutes after taking the tablet. Some patients find it convenient to take levothyroxine at bedtime (at least 3-4 hours after the last meal), which has been shown in studies to be equally effective.
Yes, Medithyrox is considered safe and essential during pregnancy. Adequate thyroid hormone levels are critical for fetal brain development and healthy pregnancy outcomes. Women with hypothyroidism often need a dose increase of 25-50% during pregnancy, particularly in the first trimester. TSH levels should be monitored regularly. Do not stop taking levothyroxine during pregnancy without medical supervision, as untreated hypothyroidism increases the risk of miscarriage, preeclampsia, and developmental issues.
Medithyrox begins working immediately upon absorption, but it takes approximately 4-6 weeks for the medication to reach full steady-state levels. You may start noticing improvements in energy and mood within 2-3 weeks. Your doctor will typically recheck your TSH level 6-8 weeks after starting or adjusting the dose. Complete symptom resolution may take several months, and some symptoms like dry skin and hair changes may take longer to improve than fatigue and weight.
If you miss a dose, take it as soon as you remember on the same day. If you do not remember until the next day, skip the missed dose and take your regular dose as scheduled. Do not take a double dose unless specifically advised by your doctor. Because levothyroxine has a long half-life of approximately 7 days, missing a single dose is unlikely to cause noticeable symptoms. However, consistently missing doses will lead to rising TSH levels and return of hypothyroid symptoms.
While different brands of levothyroxine (such as Medithyrox, Synthroid, Euthyrox, Levaxin) all contain the same active ingredient, they may differ in their excipients (inactive ingredients) and manufacturing processes. Because levothyroxine has a narrow therapeutic index, even small differences in bioavailability between brands can affect your thyroid levels. If you switch brands, your doctor should recheck your TSH after 6-8 weeks. Many endocrine societies recommend maintaining the same brand or manufacturer to ensure dose consistency.
References
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- European Medicines Agency (EMA). Levothyroxine sodium - Summary of Product Characteristics. EMA, 2024.
- U.S. Food and Drug Administration (FDA). Levothyroxine Sodium Tablets - Prescribing Information. FDA, 2024.
- World Health Organization. WHO Model List of Essential Medicines - 23rd List. WHO, 2023.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315-389. doi:10.1089/thy.2016.0457
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- Garber JR, Cobin RH, Gharib H, et al. Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22(12):1200-1235. doi:10.1089/thy.2012.0205
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. The Lancet. 2017;390(10101):1550-1562. doi:10.1016/S0140-6736(17)30703-1
- British National Formulary (BNF). Levothyroxine Sodium. NICE, 2025.
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