Synthroid (levothyroxine sodium) is the most prescribed synthetic thyroid hormone replacement medication in the United States, engineered to precisely mimic the natural thyroxine (T4) hormone produced by a healthy thyroid gland. It is the gold standard treatment for managing hypothyroidism, a condition characterized by an underactive thyroid that fails to produce sufficient hormones for the body’s metabolic needs. By providing a consistent, bioidentical hormone, Synthroid effectively restores normal thyroid levels, alleviating the debilitating symptoms of hormone deficiency and enabling patients to regain a normal metabolic state. Its standardized formulation ensures reliable dosing and predictable therapeutic outcomes, making it a cornerstone of endocrine care for millions.

Features

• Contains the active pharmaceutical ingredient levothyroxine sodium, a synthetic form of the T4 hormone.
• Available in a wide range of 12 precise dosage strengths (from 25 mcg to 300 mcg) to facilitate individualized titration.
• Manufactured with a strict quality-control process to ensure consistent potency and stability in every tablet.
• Formulated for oral administration as a scored tablet, allowing for easy splitting for dose adjustments.
• Features standardized bioavailability, providing predictable absorption and serum hormone levels.
• Backed by extensive clinical research and decades of real-world therapeutic use.

Benefits

• Effectively resolves the clinical symptoms of hypothyroidism, including fatigue, weight gain, cold intolerance, and cognitive fog.
• Restores normal metabolic function, allowing the body’s cells to produce energy efficiently.
• Helps normalize elevated thyroid-stimulating hormone (TSH) levels, a key biomarker for thyroid function.
• Prevents long-term complications of untreated hypothyroidism, such as goiter, cardiovascular disease, and myxedema coma.
• Supports healthy neurological development in fetuses of treated hypothyroid pregnant individuals.
• Improves overall quality of life, energy levels, and sense of well-being.

Common use

Synthroid is primarily indicated for the treatment of hypothyroidism, a chronic condition requiring lifelong hormone replacement therapy. It is also used as supplemental or replacement therapy in the management of euthyroid goiters, including those caused by autoimmune thyroiditis (Hashimoto’s disease). Furthermore, it is a critical component of the therapeutic regimen for thyroid hormone suppression therapy, which is used to manage certain types of thyroid cancer and prevent goiter recurrence post-thyroidectomy.

Dosage and direction

The dosage of Synthroid is highly individualized and must be determined by a healthcare provider based on the patient’s age, body weight, cardiovascular status, and the severity and etiology of their hypothyroidism. The goal of therapy is to normalize the TSH level within the laboratory reference range.

• Administration: Take Synthroid orally as a single daily dose, on an empty stomach, at least 30 to 60 minutes before breakfast.
• Consistency: Adhere to the same brand of levothyroxine throughout treatment, as switching brands may necessitate dose adjustments due to differences in bioavailability.
• Initiation: In otherwise healthy adult patients under 50, a full replacement dose may be initiated. In older patients or those with known cardiac disease, therapy typically begins with a lower dose (e.g., 25-50 mcg daily) with gradual increments every 4-6 weeks based on TSH response.
• Monitoring: Thyroid function tests (TSH) should be monitored approximately 6-8 weeks after initiating therapy, after any dose change, during pregnancy, and at least annually once a stable dose is established.

Precautions

• Cardiac Risk: Levothyroxine can exacerbate pre-existing cardiac conditions, such as angina pectoris, coronary artery disease, or arrhythmias. Over-treatment increases the risk of atrial fibrillation and osteoporosis. Dose increases must be approached cautiously in elderly patients and those with cardiac history.
• Adrenal Insufficiency: Patients with concomitant adrenal insufficiency (Addison’s disease) must have it treated with a glucocorticoid prior to initiating thyroid hormone replacement to prevent an adrenal crisis.
• Malabsorption: Conditions that affect absorption (e.g., celiac disease, lactose intolerance) or recent GI surgery may necessitate higher doses. Dosing intervals may need to be separated from other medications and supplements known to interfere with absorption.
• Pregnancy: Hypothyroidism during pregnancy requires careful management. Dose requirements frequently increase during pregnancy (often by 25-50%) and must be monitored closely. Postpartum, the dose typically returns to pre-pregnancy levels.

Contraindications

Synthroid is contraindicated in patients with:

• Uncorrected adrenal insufficiency.
• Untreated thyrotoxicosis (overactive thyroid).
• Acute myocardial infarction uncorrected by cardiology.
• Known hypersensitivity to any of the inactive ingredients in the formulation.

Possible side effect

When dosed appropriately, Synthroid is generally well-tolerated as it is replacing a deficient hormone. Side effects are almost exclusively associated with excessive dosing (iatrogenic hyperthyroidism). Signs and symptoms of overdose include:

• Palpitations, tachycardia, arrhythmias, angina pectoris.
• Tremors, nervousness, insomnia, anxiety, irritability.
• Headache, increased appetite, heat intolerance, excessive sweating.
• Fever, menstrual irregularities, weight loss, diarrhea.
• Temporary hair loss may occur during the initial months of therapy as the body’s metabolism readjusts.

Drug interaction

Numerous medications and supplements can alter the absorption or metabolism of levothyroxine, necessitating dose adjustments and careful timing of administration.

• Absorption Interference: Calcium carbonate, iron supplements, aluminum hydroxide (in antacids), sucralfate, bile acid sequestrants (cholestyramine), and proton pump inhibitors can significantly impair absorption. Administer Synthroid at least 4 hours apart from these agents.
• Metabolism Alteration: Estrogens and estrogen-containing oral contraceptives can increase thyroxine-binding globulin levels, potentially increasing levothyroxine requirements. Drugs like phenytoin, carbamazepine, and rifampin can increase the hepatic metabolism of levothyroxine, decreasing its efficacy.
• Potentiation: Levothyroxine can potentiate the effects of anticoagulant drugs (e.g., warfarin), increasing the risk of bleeding. INR should be monitored closely.

Missed dose

If a dose is missed, it should be taken as soon as it is remembered on that same day. If it is not remembered until the next day, the missed dose should be skipped, and the regular dosing schedule resumed. Do not double the next dose to make up for a missed dose. Consistency is more important than catching up on a single missed dose, which could lead to transient over-replacement.

Overdose

An overdose of Synthroid manifests as symptoms of hyperthyroidism, including tachycardia, arrhythmia, cardiac arrest, headache, irritability, fever, vomiting, and confusion. Signs and symptoms may not appear for several days after ingestion. Management is symptomatic and supportive. Beta-blockers may be used to control tachycardia and other adrenergic symptoms. Treatment should be guided by the patient’s clinical status.

Storage

Store Synthroid tablets at controlled room temperature, 20°C to 25°C (68°F to 77°F), in their original blister packaging or bottle to protect from light and moisture. Keep the container tightly closed. Do not store in the bathroom or other areas with high humidity. Keep out of reach of children and pets.

Disclaimer

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or before starting any new treatment. Never disregard professional medical advice or delay in seeking it because of something you have read here. Individual patient needs and responses to medication vary.

Reviews

• “As an endocrinologist with over 20 years of practice, I find Synthroid to be a reliable and predictable agent for managing hypothyroidism. Its wide range of strengths allows for precise titration, which is critical for achieving optimal TSH levels and patient well-being.” – Dr. E. Lawson, MD, Endocrinology
• “After years of struggling with unexplained fatigue and brain fog, starting on Synthroid was life-changing. It took a few months to get the dose exactly right, but the consistent quality of the medication has kept my levels stable for years. I finally feel like myself again.” – Patient, 48
• “The bioavailability and consistency of brand-name Synthroid are its greatest assets from a clinical pharmacology perspective. While generic levothyroxine is available, the narrow therapeutic index of this drug means that even minor variations can impact a patient’s TSH. For my patients on a stable dose, I strongly recommend staying with the same manufacturer.” – Clinical Pharmacist
• “Managing my hypothyroidism during pregnancy was a concern, but my doctor closely monitored my levels and adjusted my Synthroid dose accordingly. I’m grateful for a medication that allowed me to have a healthy pregnancy and a healthy baby.” – Patient, 32