Synthroid 25 mcg Tablets

Brand Name

Synthroid is the widely acknowledged brand name under which this medication is marketed, celebrated in clinical settings for its critical role in managing thyroid-related conditions. It has earned a reputation as a cornerstone therapy for healthcare providers addressing hormonal imbalances.

International Nonproprietary Name (INN)

The active component of this drug is identified by its International Nonproprietary Name (INN) as levothyroxine sodium. This synthetic thyroid hormone is precisely formulated to replicate the body’s natural thyroid function.

Forms of Release

Synthroid is available exclusively as oral tablets, offered in a broad range of strengths — 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg — to accommodate individualized dosing needs. These tablets are designed for daily administration, providing a consistent and reliable delivery method for patients requiring thyroid hormone replacement or supplementation. No alternative forms like capsules or solutions are marketed, focusing on this precise oral route.

Composition

The primary active ingredient in Synthroid is levothyroxine sodium, which drives its therapeutic effects. Alongside this essential substance, the tablets include several inactive components to ensure stability, bioavailability, and ease of ingestion. These comprise acacia, lactose monohydrate, and magnesium stearate as fillers and stabilizers, with confectioner’s sugar and povidone aiding in tablet formation. Colorants like FD&C Yellow No. 6 or Blue No. 1 vary by strength for identification, ensuring no therapeutic impact. These auxiliary elements collectively maintain the drug’s consistency and support its administration profile.

Pharmacologic Properties

Pharmacodynamics

This medication exerts its influence by mimicking the actions of endogenous thyroid hormones. Levothyroxine sodium replicates thyroxine (T4), which is converted peripherally into triiodothyronine (T3), the more active form, via deiodination in tissues like the liver and kidneys. It regulates metabolism by binding to nuclear thyroid hormone receptors, influencing gene expression to enhance oxygen consumption, protein synthesis, and carbohydrate metabolism. This action restores normal metabolic rates in hypothyroidism, reduces goiter size by suppressing thyroid-stimulating hormone (TSH), and supports overall energy balance. Its precise hormonal replacement offers a steady correction of thyroid deficiency, normalizing physiological functions effectively.

Pharmacokinetics

The pharmacokinetic behavior of Synthroid describes how levothyroxine sodium is processed within the body. After oral administration, it is absorbed primarily in the jejunum and ileum, with bioavailability ranging from 40% to 80%, reduced by food (optimal when taken fasting). Peak plasma concentrations occur within 6 to 8 hours. It is 99% bound to plasma proteins, mainly thyroxine-binding globulin (TBG), with a volume of distribution of 10–12 liters, reflecting thyroid hormone distribution. Metabolism occurs in the liver, kidneys, and other tissues via deiodination to T3 or inactive reverse T3, with conjugation forming glucuronides and sulfates. Excretion is primarily through urine (80%) and some via bile, with a half-life of 6–7 days in euthyroid states, prolonged in hypothyroidism (9–10 days) and shortened in hyperthyroidism (3–4 days), necessitating stable dosing adjustments.

Indications for Use

This medication is prescribed for conditions involving thyroid hormone deficiency or suppression needs, leveraging its ability to restore hormonal balance across various clinical contexts.

Diseases and Conditions

Synthroid is indicated for hypothyroidism treatment in adults and children, including primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) forms, restoring metabolic function. It is also used for TSH suppression in thyroid cancer post-surgery, preventing recurrence, and in nontoxic goiter to reduce gland size. Additionally, it supports myxedema coma management as an adjunct, addressing severe hypothyroid states. Its precise hormonal action makes it essential for these thyroid-related disorders.

Contraindications

Despite its benefits, certain conditions render this medication unsuitable due to significant risks, requiring thorough screening before initiation.

Conditions Prohibiting Use

Synthroid is contraindicated in patients with known hypersensitivity to levothyroxine or any tablet components, as reactions could range from mild rashes to severe anaphylaxis. It is prohibited in untreated adrenal insufficiency, as it may precipitate an adrenal crisis by increasing metabolic demand. Acute myocardial infarction, untreated thyrotoxicosis, or uncorrected hyperthyroidism also bar its use due to cardiovascular strain risks. These restrictions ensure safety by preventing exacerbation of underlying conditions.

Method of Administration and Dosage

How to Take

This medication is administered orally as tablets, swallowed whole with a full glass of water. It should be taken on an empty stomach, ideally 30–60 minutes before breakfast, to maximize absorption, avoiding food, coffee, or calcium-rich items that impair uptake. Consistency in timing—preferably morning—supports steady hormone levels, and patients should not crush or split tablets unless directed, preserving the intended dose accuracy.

Dosages for Adults and Children

Dosing is individualized based on age, condition, and TSH levels. For hypothyroidism in adults under 50 without cardiac disease, Synthroid starts at 1.6 mcg/kg/day (e.g., 100–125 mcg/day), adjusted every 4–6 weeks by 12.5–25 mcg to normalize TSH. Older adults or those with cardiac risk begin at 25–50 mcg/day, titrating slowly. Children with congenital hypothyroidism start at 10–15 mcg/kg/day (e.g., 25–50 mcg for infants), reducing with age (e.g., 2–3 mcg/kg/day for teens). Thyroid cancer suppression uses 2 mcg/kg/day, targeting low TSH. Dosing precision ensures optimal replacement across populations.

Dose Adjustment in Specific Conditions

Adjustments are needed in organ impairment or special states. In mild to moderate renal dysfunction (creatinine clearance 30–80 mL/min), no change is typically required, with severe cases (below 30 mL/min) monitored closely without routine reduction due to minimal renal impact. Mild to moderate hepatic impairment (Child-Pugh A or B) uses standard doses, with severe cases (Child-Pugh C) suggesting caution and possible lower starting doses (e.g., 25 mcg/day). Elderly or cardiac patients start low (12.5–25 mcg/day), with gradual increases, guided by TSH and tolerability, balancing efficacy and safety.

Side Effects

This medication can provoke a range of adverse reactions, typically from dosing imbalances, and patients should be educated to recognize and address them promptly.

Possible Adverse Reactions

Common effects mimic hyperthyroidism from excess dosing—palpitations, sweating, or nervousness—resolving with adjustment. Fatigue, headache, or insomnia may occur, often transient. Gastrointestinal upset—nausea or diarrhea—is less frequent, while weight loss or heat intolerance reflects metabolic shifts. Rare but serious effects include arrhythmias, angina, or osteoporosis with prolonged overuse, particularly in postmenopausal women. Allergic reactions (e.g., rash) or rare alopecia also occur, with regular TSH monitoring mitigating these risks effectively.

Overdose

Symptoms of Overdose

Excessive intake of Synthroid can lead to thyrotoxicosis symptoms requiring immediate attention. These include rapid heartbeat, sweating, or tremors from heightened metabolism, alongside anxiety, irritability, or confusion. Fever, chest pain, or shortness of breath may signal cardiovascular strain, while severe cases could involve seizures or coma, though rare. Symptoms escalate with dose and duration, necessitating swift recognition.

First Aid Measures

In an overdose scenario, rapid response is key. If the patient is alert and ingestion was recent, contact emergency services or poison control without inducing vomiting unless advised. Maintain airway and vital signs while awaiting help. In medical settings, beta-blockers (e.g., propranolol) manage symptoms, with activated charcoal considered within an hour of ingestion if significant. Supportive care—hydration, monitoring—resolves most cases, with dialysis ineffective due to protein binding.

Drug Interactions

Effects on Other Medications

This medication can influence co-administered drugs via absorption or metabolism effects. It increases warfarin’s anticoagulant effect by enhancing metabolism, requiring dose adjustments. Antacids, calcium, or iron reduce its absorption, needing a 4-hour separation. Cholestyramine or colestipol bind it, lowering levels, while rifampin or carbamazepine accelerate clearance, potentially raising TSH. Diabetes drugs may need adjustment due to altered glucose metabolism, ensuring safe co-therapy with monitoring.

Compatibility with Alcohol and Food

Alcohol poses no direct interaction with Synthroid, though moderation avoids metabolic interference. Food, especially calcium-rich or high-fiber items, reduces absorption, making fasting administration optimal, with no strict dietary limits beyond timing consistency for efficacy.

Special Precautions

Use During Pregnancy and Breastfeeding

This medication is Category A, safe and essential in pregnancy to maintain euthyroidism, with doses often increased (e.g., 25–50% more) due to higher demand, monitored by TSH. Levothyroxine enters breast milk in trace amounts; breastfeeding is safe and recommended, supporting maternal and infant health without notable effects.

Impact on Driving and Operating Machinery

Synthroid rarely affects alertness unless overdosed, causing nervousness or palpitations. Patients should avoid driving or machinery if symptomatic until stabilized, ensuring safety.

Considerations for Elderly and Pediatric Patients

In elderly patients, dosing starts low (12.5–25 mcg/day) due to cardiac sensitivity, with slow titration and TSH checks. In children, congenital hypothyroidism uses 10–15 mcg/kg/day, adjusted with growth (e.g., 2–3 mcg/kg/day in teens), monitored for development, ensuring tailored care across ages.