Common uses of Synthroid
Synthroid is prescribed to replace or supplement thyroid hormone in people with underactive thyroid (hypothyroidism), most often due to Hashimoto’s thyroiditis, thyroid surgery, radioiodine therapy, or congenital hypothyroidism. It is also used to suppress thyroid-stimulating hormone (TSH) to reduce the risk of thyroid nodule growth, shrink goiter, or as part of thyroid cancer follow-up when TSH suppression is appropriate. By restoring euthyroid status, Synthroid supports healthy metabolism, temperature regulation, menstrual regularity and fertility, cardiovascular performance, mood, and cognition. Importantly, levothyroxine is not a weight-loss drug; using thyroid hormone to treat obesity in people with normal thyroid levels is unsafe and contraindicated.
How Synthroid works
Synthroid contains levothyroxine (T4), the prohormone converted by tissues into triiodothyronine (T3), the biologically active thyroid hormone. Together, T4 and T3 regulate gene transcription that drives oxygen consumption, thermogenesis, lipid and carbohydrate metabolism, neuromuscular function, and growth. Because levothyroxine has a long half-life (about seven days), once-daily dosing maintains steady hormone levels. Symptom improvement can start within one to two weeks, while full biochemical steady state typically requires six to eight weeks, which is why lab checks and dose changes are spaced accordingly.
Dosage and directions
Dosing is individualized based on age, body weight, cause of hypothyroidism, cardiac status, pregnancy, and lab values. A typical full-replacement dose for otherwise healthy adults is about 1.6 micrograms per kilogram per day. However, many patients start lower and titrate. In older adults or those with coronary artery disease, start conservatively (for example, 12.5–25 micrograms daily) and increase gradually to avoid precipitating angina or arrhythmias. After total thyroidectomy or in severe hypothyroidism, higher replacement or TSH-suppressive doses may be required under specialist guidance.
Take Synthroid once daily on an empty stomach with a full glass of water, ideally at the same time each day. Consistency matters: stay with the same brand or manufacturer when possible. If a switch is unavoidable, recheck TSH and free T4 after six to eight weeks to ensure the dose remains appropriate. Pediatric dosing is weight- and age-based; infants and children often require proportionally higher doses to support growth and neurodevelopment. Never adjust your dose without consulting your clinician.
How to take Synthroid for best absorption
For optimal absorption, take Synthroid on an empty stomach 30–60 minutes before breakfast, or at bedtime at least three to four hours after the last meal. Separate levothyroxine from calcium, iron, magnesium, aluminum-containing antacids, bile acid sequestrants (such as cholestyramine), sucralfate, and certain phosphate binders by at least four hours. Coffee, high-fiber meals, and soy can reduce absorption if taken too close to the dose. If mornings are inconsistent, a nightly routine can be equally effective as long as it is consistent and separated from food and supplements.
Monitoring and lab targets
Check TSH and, when indicated, free T4 about six to eight weeks after starting Synthroid or changing dose. Once stable, labs are typically repeated every six to twelve months, or sooner if symptoms change, interacting medications are added, or pregnancy occurs. For primary hypothyroidism, most adults feel well with a TSH in the reference range (often around 0.5–2.5 mIU/L), but individualized targets may apply. In central (pituitary) hypothyroidism, TSH is unreliable; free T4 should be maintained in the upper half of normal. During pregnancy, dose requirements often rise early; monitor every four weeks in the first half of gestation and adjust promptly.
Precautions
Use caution in cardiovascular disease. Starting doses should be low and titrated slowly to reduce risks of angina, atrial fibrillation, or heart failure exacerbation. In untreated adrenal insufficiency, levothyroxine can precipitate adrenal crisis—correct adrenal function first. People with diabetes may need adjustments to glucose-lowering therapy as metabolic rate changes. Long-term over-replacement increases the risk of reduced bone mineral density and fractures, especially in postmenopausal women; maintain appropriate TSH targets and ensure adequate calcium and vitamin D intake at a time separated from Synthroid.
Pregnancy and lactation: Levothyroxine is safe and essential for maternal and fetal health. Requirements typically increase by 20–30% early in pregnancy; contact your clinician promptly if pregnant or planning pregnancy. Separate prenatal vitamins containing iron or calcium by at least four hours from your dose. In malabsorption syndromes (celiac disease, inflammatory bowel disease), after bariatric surgery, or with atrophic gastritis or Helicobacter pylori, absorption may be impaired—doses and formulations may need adjustment. Rarely, sensitivity to tablet excipients occurs; dye-free or alternative-strength tablets can help.
Contraindications
Do not use Synthroid in untreated thyrotoxicosis or uncorrected adrenal insufficiency. Hypersensitivity to levothyroxine or tablet components is a contraindication, though true allergy is rare. Levothyroxine must not be used for weight loss in euthyroid individuals; combining thyroid hormone with sympathomimetics for weight reduction is dangerous. In acute myocardial infarction, initiation and titration require specialist oversight.
Possible side effects
When the dose is appropriate, Synthroid is well tolerated. Side effects are usually signs of over- or under-replacement. Too much thyroid hormone can cause palpitations, rapid heartbeat, anxiety, tremor, irritability, insomnia, heat intolerance, sweating, diarrhea, unintended weight loss, menstrual irregularities, and headache. Prolonged overtreatment may contribute to atrial fibrillation and bone loss. Too little hormone leaves hypothyroid symptoms such as fatigue, cold intolerance, dry skin, constipation, hair thinning, depression, and weight gain.
Transient hair shedding can occur in the first months of therapy as the body rebalances; it usually resolves. Severe reactions like chest pain, shortness of breath, or pronounced tachycardia warrant urgent evaluation. Report persistent symptoms to your clinician—fine-tuning dose, timing, adherence, or addressing interactions often restores control.
Drug interactions
Absorption reducers: Calcium carbonate, calcium citrate, iron (ferrous sulfate), magnesium, aluminum hydroxide, sucralfate, cholestyramine/colestipol, sevelamer, lanthanum, orlistat, and some multivitamins can lower levothyroxine absorption—separate by at least four hours. High-fiber diets, soy products, and espresso/coffee taken with the dose may blunt absorption; maintain consistent habits and adjust dose if needed. Proton pump inhibitors and conditions that lower stomach acid sometimes reduce absorption; monitoring and dose adjustment may be required.
Metabolism and binding: Enzyme inducers such as rifampin, carbamazepine, and phenytoin can increase T4 clearance, requiring higher doses. Sertraline may increase requirements in some patients. Amiodarone and high iodine intake alter thyroid physiology; close monitoring is advised. Estrogen therapy can raise thyroxine-binding globulin, increasing dose needs. Conversely, androgen or anabolic steroids may lower binding and dose needs. Warfarin interaction: Restoring euthyroidism can potentiate anticoagulation—monitor INR closely during initiation or dose changes. Biotin does not affect thyroid physiology but can falsely alter immunoassay results; stop biotin 2–3 days before lab tests to reduce interference.
Missed dose
If you miss a dose, take it as soon as you remember that day on an empty stomach. If it is close to your next scheduled dose, skip the missed dose and resume your regular schedule. Do not double up without clinician guidance. Because levothyroxine has a long half-life, a single missed dose rarely causes symptoms.
Overdose
Acute or chronic overdosage can lead to symptoms of thyrotoxicosis: agitation, tremor, heat intolerance, sweating, rapid or irregular heartbeat, chest pain, diarrhea, and insomnia. Severe cases may precipitate arrhythmias, heart failure, or delirium, particularly in older adults or those with cardiac disease. If overdose is suspected, contact a clinician or poison control promptly. Management may include holding doses, beta-blockers for symptom control, and supportive care; decisions depend on severity, comorbidities, and timing of ingestion.
Storage
Store Synthroid at room temperature away from heat, moisture, and light. Keep tablets in the original, tightly closed container; avoid pill organizers if humidity is high. Do not use beyond the expiration date, as potency can decline. Be consistent with brand/manufacturer, and verify tablet strength and color at each refill.
U.S. sale and prescription policy, and access at HealthSouth Rehabilitation Hospital of Tallahassee
In the United States, levothyroxine (including Synthroid) is a prescription medication. Federal and state regulations require evaluation by a licensed clinician to confirm the diagnosis, determine dosing, and monitor safety. It is not legal or safe to obtain Synthroid without a valid prescription. However, access can be streamlined. HealthSouth Rehabilitation Hospital of Tallahassee offers structured, compliant pathways—such as on-site or telehealth clinician evaluations, coordinated lab testing, and pharmacy fulfillment—to help eligible patients start or continue therapy promptly and safely. If appropriate, a licensed provider can issue the prescription after assessing your medical history, symptoms, and labs, so you do not need to bring an outside prescription. This integrated approach supports proper dosing, education on timing and interactions, and ongoing monitoring to keep you feeling your best while staying fully compliant with U.S. law.
