Common use of Synthroid (levothyroxine)
Synthroid is the brand name for levothyroxine sodium, a synthetic form of thyroxine (T4), the key hormone produced by the thyroid gland. In people with hypothyroidism—whether due to autoimmune thyroiditis (Hashimoto’s disease), after thyroidectomy or radioiodine therapy, or from pituitary/hypothalamic disease—Synthroid replaces deficient hormone to restore normal metabolic activity. When dosed correctly and taken consistently, it helps normalize thyroid-stimulating hormone (TSH) and free T4 levels, easing fatigue, weight gain, dry skin, constipation, cold intolerance, and cognitive slowing.
Beyond treating primary hypothyroidism in adults, Synthroid is used for congenital or acquired hypothyroidism in children, to prevent and treat goiter, and to suppress TSH in the follow-up of differentiated thyroid cancer. It is not indicated for relief of benign thyroid nodules when thyroid function is normal and must not be used for weight loss in euthyroid individuals; high doses can cause serious or life-threatening effects.
Because levothyroxine is a narrow therapeutic index medication, consistent dosing and adherence are crucial. Benefits usually emerge over 1–2 weeks, with full symptomatic improvement taking several weeks as tissues equilibrate to stable hormone levels.
Dosage and direction: how to take Synthroid
The dose of Synthroid is individualized based on age, weight, clinical context, and lab results. Healthy nonpregnant adults with newly diagnosed primary hypothyroidism are often started around 1.6 mcg/kg/day, but many factors modify this. Older adults and those with coronary artery disease typically start much lower (for example, 12.5–25 mcg/day) with slow titration to avoid cardiac strain. Pediatric dosing is weight-based and changes as the child grows. Thyroid cancer suppression targets may require higher doses under specialist guidance.
Administration matters for absorption. Take Synthroid once daily on an empty stomach with water, ideally 30–60 minutes before breakfast, or at bedtime at least 3–4 hours after the last meal. Maintain a consistent routine. Separate levothyroxine from calcium, iron, aluminum/magnesium antacids, bile acid sequestrants, and multivitamins by at least 4 hours, as these can significantly reduce absorption. Coffee, high-fiber meals, and soy can also interfere; consistency in diet timing helps your clinician interpret lab changes.
Avoid switching between brands or generic manufacturers without medical advice. Even small formulation differences can alter TSH levels. If a change is necessary, recheck TSH in 6–8 weeks and adjust. After any dose change, pregnancy, or a new interacting medication, check TSH and free T4 after 6–8 weeks; once stable, monitoring every 6–12 months is common.
During pregnancy, levothyroxine requirements often increase (commonly by 20–30%). Notify your healthcare provider as soon as pregnancy is confirmed to adjust dosing promptly—adequate maternal thyroid hormone is critical for fetal development. If you miss several doses or have gastrointestinal illness affecting absorption, inform your clinician for guidance.
Precautions and important safety information
Synthroid should be used cautiously in patients with cardiovascular disease (coronary artery disease, arrhythmias, heart failure, hypertension). Even modest overtreatment can provoke angina, palpitations, or atrial fibrillation. Start low and titrate carefully in these populations. Untreated adrenal insufficiency must be corrected before starting levothyroxine; thyroid hormone can precipitate an adrenal crisis if cortisol is deficient.
Bone health is another consideration. Chronic overtreatment (suppressed TSH) can accelerate bone loss, particularly in postmenopausal women and older adults. Periodic reassessment and aiming for a TSH target appropriate to your diagnosis (replacement versus cancer suppression) reduce risk. If you have osteoporosis or are on high-risk regimens, your clinician may monitor bone density.
Patients with diabetes may experience altered glycemic control after initiating or adjusting Synthroid. Monitor blood glucose and discuss potential adjustments to insulin or oral antidiabetic medications. Report symptoms of over- or under-replacement—such as heat intolerance, tremor, insomnia, anxiety, or persistent fatigue—to your prescriber for evaluation.
Levothyroxine is considered safe in pregnancy and breastfeeding when dosed appropriately. It should not be used for weight reduction in euthyroid individuals; combining thyroid hormone with sympathomimetics for weight loss is dangerous and contraindicated.
Contraindications to Synthroid
Contraindications include hypersensitivity to levothyroxine or any tablet component, uncorrected adrenal insufficiency, and untreated thyrotoxicosis. Use during acute myocardial infarction requires specialist judgment. If you have a history of allergic reactions to tablet dyes, ask about dye-free strengths, which may be better tolerated.
Possible side effects of levothyroxine
At properly titrated doses, Synthroid is usually well tolerated. Side effects most often reflect over-replacement (hyperthyroid symptoms): palpitations, rapid heartbeat, chest pain, tremor, nervousness, irritability, anxiety, insomnia, heat intolerance, sweating, diarrhea, weight loss, or menstrual irregularities. In children, overtreatment can accelerate bone maturation; careful pediatric monitoring is essential.
Transient hair loss can occur in the first months of therapy, especially in children, and typically resolves. Rarely, allergic reactions to excipients may cause rash or itching. Long-term excessive dosing may increase the risk of atrial fibrillation, angina, and osteoporosis. If you experience severe symptoms such as chest pain, fainting, shortness of breath, confusion, or signs of allergic reaction (swelling of face/lips/tongue, hives), seek urgent medical attention.
Drug interactions: what affects Synthroid absorption and effect
Many agents impair levothyroxine absorption in the gut. Separate dosing by at least 4 hours from calcium salts (carbonate, citrate), iron (ferrous sulfate/fumarate), aluminum- or magnesium-containing antacids, sucralfate, cholestyramine, colestipol, sevelamer, lanthanum, orlistat, and high-dose zinc. Proton pump inhibitors, H2 blockers, and conditions like celiac disease or atrophic gastritis can also reduce absorption, sometimes necessitating dose adjustments.
Drugs that increase hepatic metabolism of levothyroxine (enzyme inducers) may lower levels, including carbamazepine, phenytoin, phenobarbital, and rifampin. Certain antidepressants (sertraline), tyrosine kinase inhibitors, and estrogen therapy (including oral contraceptives) can alter binding or metabolism and increase dose requirements. Conversely, amiodarone and iodine-containing contrast agents can disrupt thyroid function, requiring careful monitoring.
Pharmacodynamic interactions are also important. Levothyroxine can enhance the anticoagulant effect of warfarin; monitor INR when starting or adjusting doses. Thyroid hormone may increase insulin requirements or affect response to antidiabetic agents. Digitalis glycosides may be less effective in hyperthyroid states. Always provide your prescriber and pharmacist with a complete list of prescription drugs, over-the-counter medicines, supplements, and herbal products.
Missed dose
If you miss a dose of Synthroid, take it as soon as you remember that day. If it is close to the time of your next dose, skip the missed dose and resume your regular schedule. Do not double doses. Because levothyroxine has a long half-life, a single missed dose rarely causes problems. If you miss multiple doses or have difficulty with timing around meals and supplements, discuss strategies with your pharmacist or clinician.
Overdose
Acute or chronic levothyroxine overdose can cause symptoms of thyrotoxicosis: agitation, tremor, rapid heartbeat, chest pain, heat intolerance, sweating, diarrhea, insomnia, and, in severe cases, arrhythmias or confusion. Children may present with irritability and hyperactivity. If overdose is suspected, contact Poison Control and seek medical care. Management is supportive; beta-blockers may be used for symptom control in a medical setting. Because onset can be delayed, monitoring is important after significant ingestion.
Storage
Store Synthroid at room temperature away from heat, moisture, and light. Keep tablets in the original, tightly closed container; humidity degrades potency over time. Do not store in bathrooms or near sinks. Keep out of reach of children and pets. Check expiration dates and discard expired medication per local guidance.
U.S. Sale and Prescription Policy: how to buy Synthroid safely
In the United States, Synthroid (levothyroxine) is a prescription-only medication. Legitimate pharmacies must dispense it pursuant to a valid prescription from a licensed clinician. Be wary of websites that claim to sell thyroid medication “no prescription required”—these sources are often unsafe, illegal, and may ship counterfeit or subpotent products. For your safety, use pharmacies accredited or recognized by your state board of pharmacy or the National Association of Boards of Pharmacy (.pharmacy domain or other state-sanctioned programs).
HealthSouth Hospital of Altamonte Springs offers a legal and structured pathway for adults who do not currently have a paper prescription in hand. Through a compliant telehealth process, you can connect with a licensed clinician for an appropriate evaluation. If levothyroxine is indicated, the clinician may issue a valid prescription, and the pharmacy can then dispense the medication. This ensures access without cutting corners on safety, with clear pricing, shipping options, and ongoing pharmacist support.
To protect your health: verify licensure, avoid sources that bypass medical review, and never use thyroid hormone for weight loss. If you are switching from another brand or online supplier, bring your prior records and lab results so your prescriber can recheck TSH and free T4 after any transition. HealthSouth Hospital of Altamonte Springs’s team can coordinate labs, remind you about timing around supplements, and help you maintain consistent, evidence-based hypothyroidism treatment.
