Common uses of Synthroid (levothyroxine)
Synthroid replaces or supplements thyroxine (T4), the hormone your thyroid normally produces. Its primary use is treating hypothyroidism—whether due to Hashimoto’s thyroiditis, thyroid surgery, radioactive iodine ablation, congenital thyroid disorders, or medications that suppress the thyroid. By normalizing thyroid hormone levels, patients typically see improvements in energy, mood, cognition, hair/skin health, bowel regularity, and weight trends over time.
Clinicians also use Synthroid to suppress thyroid-stimulating hormone (TSH) in select situations, such as differentiated thyroid cancer follow-up or certain nodular goiters, where lower TSH can reduce stimulation of remaining thyroid tissue. TSH suppression targets are personalized and revisited regularly to balance benefit with potential risks like bone loss or heart strain.
In pregnancy, adequate thyroid hormone is crucial for maternal health and fetal neurodevelopment. Many patients need a higher dose during pregnancy, with close TSH monitoring each trimester. After delivery, dosing usually returns toward pre-pregnancy levels. Treating hypothyroidism before and during pregnancy reduces miscarriage risk, preeclampsia, and developmental complications.
Importantly, Synthroid is not a weight-loss drug and should never be used for obesity in euthyroid (normal thyroid) individuals. Using thyroid hormone to accelerate metabolism without medical indication can be dangerous, increasing the risk of heart rhythm problems, bone loss, and other adverse effects.
Dosage and direction for taking Synthroid
Your dose of Synthroid is individualized. Clinicians typically use body weight, age, cardiovascular status, and the cause of hypothyroidism to estimate a starting dose, then adjust using TSH and free T4 blood tests. Most healthy, younger adults with overt hypothyroidism eventually require about 1.6 micrograms per kilogram per day; older adults and people with heart disease often start much lower to avoid overstimulation of the heart, titrating slowly every 6–8 weeks.
Take Synthroid once daily on an empty stomach with water, ideally first thing in the morning. Wait at least 30–60 minutes before eating breakfast or drinking coffee. Alternatively, some patients dose consistently at bedtime, 3–4 hours after the last meal. The key is consistency—same time, same way—to minimize absorption variability and maintain steady thyroid hormone levels.
Separate Synthroid from certain supplements and medications that impair absorption: calcium, iron, magnesium, multivitamins, antacids containing aluminum or calcium, sucralfate, bile acid sequestrants (cholestyramine, colestipol), sevelamer, and orlistat. A general rule is to space these at least 4 hours apart. Discuss your full medication list with your clinician to design a schedule that works.
Be brand-consistent when possible. Different levothyroxine products can have small potency differences that matter for sensitive patients. If a switch in brand or generic manufacturer occurs, recheck TSH in 6–8 weeks. Likewise, recheck labs after any notable change in diet (for example, introducing significant soy or high-fiber intake), new medications, or changes in adherence.
Pediatric dosing is carefully weight-based and changes as children grow. Infants with congenital hypothyroidism need prompt treatment to support normal brain development, with close monitoring. In pregnancy, most patients need a dose increase—sometimes as early as a positive test—and TSH checks every 4–6 weeks. After delivery, doses are usually reduced to pre-pregnancy levels with follow-up labs.
Never double up or self-adjust your dose without guidance. If your TSH is out of range or you have symptoms of over- or under-treatment, contact your healthcare provider to plan a safe adjustment and follow-up testing.
Precautions before and during Synthroid therapy
Cardiovascular considerations are paramount. Thyroid hormone increases heart rate and contractility. People with coronary artery disease, atrial fibrillation, or heart failure usually start at lower doses and titrate gradually to avoid angina or arrhythmias. Report palpitations, chest pain, or shortness of breath promptly.
Untreated adrenal insufficiency must be corrected before starting levothyroxine. Thyroid hormone can accelerate cortisol metabolism and precipitate adrenal crisis if cortisol is deficient. If there is clinical suspicion—such as hyperpigmentation, salt craving, low blood pressure—clinicians may evaluate adrenal function first.
Metabolic conditions like diabetes can be affected. As hypothyroidism resolves, insulin and oral hypoglycemic requirements may shift, and anticoagulants (e.g., warfarin) can have enhanced effect when thyroid hormone is increased. Close monitoring during dose changes helps keep glucose and INR in range.
Additional cautions include osteoporosis risk (especially with long-term TSH suppression), seizure disorders, and malabsorption syndromes (celiac disease, inflammatory bowel disease) that may alter levothyroxine needs. High-dose biotin supplements can interfere with thyroid blood tests; stop biotin 2–3 days before lab work, or follow your lab’s instructions.
Contraindications to Synthroid
Synthroid is contraindicated in uncorrected adrenal insufficiency and untreated thyrotoxicosis (hyperthyroidism). Hypersensitivity to any component of the formulation is also a contraindication, though true allergy to levothyroxine itself is exceedingly rare.
Use levothyroxine with caution and close supervision in patients with recent myocardial infarction, active cardiac ischemia, or arrhythmias, typically starting very low and titrating carefully if replacement is necessary.
Possible side effects of Synthroid
Most side effects reflect over-replacement (too much thyroid hormone) or under-replacement (too little). Signs of excess include jitteriness, anxiety, insomnia, tremor, heat intolerance, sweating, diarrhea, unintended weight loss, hair thinning, menstrual irregularities, palpitations, or chest discomfort. If these occur, contact your clinician—dose adjustment may be needed.
Signs of insufficient dosing mirror hypothyroidism: fatigue, cold intolerance, constipation, dry skin, hair loss, depressed mood, slowed thinking, and weight gain. Work with your provider to verify adherence, timing relative to food/supplements, and whether an incremental dose increase is warranted.
Rare adverse events include atrial fibrillation (especially in older adults or those with heart disease), reduced bone mineral density with long-term excessive dosing or intentional TSH suppression, and in children, pseudotumor cerebri (headache, visual changes). Severe allergic reactions to excipients are uncommon but possible; seek urgent care for hives, swelling, or trouble breathing.
If you are pregnant or planning pregnancy, notify your clinician promptly. Appropriate dosing reduces maternal and fetal risks; undertreatment or overtreatment can both have consequences. Breastfeeding is generally compatible with Synthroid, and dosing is managed to maintain maternal euthyroidism.
Drug interactions with Synthroid (levothyroxine)
Absorption interactions are common. Take Synthroid on an empty stomach and separate it by at least 4 hours from: iron (ferrous sulfate), calcium carbonate/citrate, magnesium, aluminum-containing antacids, sucralfate, bile acid sequestrants (cholestyramine, colestipol), sevelamer, lanthanum, and orlistat. High-fiber diets, soy products, and espresso/coffee close to dosing can also reduce absorption; consistency is key.
Metabolic inducers can increase levothyroxine requirements by accelerating T4 breakdown: carbamazepine, phenytoin, phenobarbital, rifampin, and some tyrosine kinase inhibitors. If you start or stop these, recheck TSH in 6–8 weeks.
Protein-binding and hormone-binding changes affect dosing. Estrogen therapy (including pregnancy and oral contraceptives) raises thyroxine-binding globulin, often necessitating a higher Synthroid dose. Androgens can lower binding. Nephrotic syndrome and liver disease may also alter binding dynamics.
Pharmacodynamic interactions matter clinically. Levothyroxine can potentiate warfarin’s anticoagulant effect; monitor INR closely during dose changes. Conversely, it can lower serum digoxin levels and alter requirements for insulin or oral hypoglycemics as metabolism normalizes. Sertraline and some SSRIs may increase thyroid hormone requirements, and amiodarone can complicate thyroid function through iodine effects.
Lab test interference is common with high-dose biotin, which can skew TSH and free T4 results. Pause biotin before testing per lab guidance. Always provide a complete medication and supplement list to your healthcare team and pharmacist.
Missed dose instructions for Synthroid
If you miss a dose, take it as soon as you remember the same day. If it is close to your next dose, skip the missed tablet—do not double up. Because levothyroxine has a long half-life, a single missed dose rarely causes problems. Maintain your usual timing with respect to meals and interacting supplements, and note the miss for discussion if symptoms recur.
Overdose: signs and what to do
Too much Synthroid can produce symptoms of hyperthyroidism: nervousness, tremor, rapid or irregular heartbeat, chest pain, shortness of breath, heat intolerance, diarrhea, sweating, and insomnia. Severe overdose can provoke arrhythmias or, rarely, thyroid storm-like presentations.
If you suspect overdose—especially with chest pain, severe palpitations, confusion, or fainting—seek emergency care or call 911. For non-emergent concerns, contact your clinician or poison control (in the U.S., 1-800-222-1222) for guidance. Do not self-correct with abrupt discontinuation without medical input unless instructed.
Storage recommendations
Store Synthroid at room temperature, away from heat, light, and moisture. Bathrooms and hot cars can degrade potency. Keep tablets in their original container with desiccant, tightly closed, and out of reach of children and pets. Do not use past the expiration date; potency matters for maintaining stable thyroid levels.
U.S. sale and prescription policy for Synthroid
In the United States, Synthroid (levothyroxine) is a prescription-only medication. Buying Synthroid without prescription is unsafe and may be illegal. Proper diagnosis, dosing, and monitoring require a licensed clinician and periodic thyroid function tests to keep you in range and to avoid cardiac or bone complications from over- or under-treatment.
If convenience is a concern, many systems offer telehealth or same-day clinics to evaluate symptoms, order TSH and free T4 labs, and prescribe Synthroid when appropriate. These legitimate options give you timely access to care without compromising safety or regulatory requirements.
Geisinger HealthSouth provides a legal, structured care pathway: you can connect with licensed clinicians for evaluation, obtain necessary lab testing, and, if indicated, receive a valid prescription with coordinated pharmacy fulfillment and follow-up monitoring. This approach protects you from the risks of no-prescription sales and ensures that your dose is personalized, your interactions are reviewed, and your therapy remains safe and effective over time.
