Common uses of Synthroid (levothyroxine)
Synthroid is levothyroxine sodium, a synthetic version of thyroxine (T4) that replaces the hormone your thyroid would ordinarily produce. Its main indication is hypothyroidism—whether caused by Hashimoto’s thyroiditis, thyroidectomy, radioiodine treatment, congenital deficiency, or medicines that suppress thyroid function. By restoring thyroid hormone to the appropriate range, many people notice gradual improvements in energy, mood, concentration, hair and skin, bowel regularity, and weight trajectory.
Clinicians may also use Synthroid to deliberately lower thyroid-stimulating hormone (TSH) in specific scenarios, including follow-up for differentiated thyroid cancer or certain nodular goitres, where reduced TSH helps limit stimulation of residual thyroid tissue. Suppression targets are tailored to the individual and reviewed routinely to balance potential benefits with risks such as reduced bone density or cardiac strain.
During pregnancy, sufficient thyroid hormone is vital for the mother and the developing baby’s brain. Many patients require a higher dose while pregnant, with close TSH monitoring each trimester. After birth, dosing typically trends back toward pre-pregnancy levels. Treating hypothyroidism before conception and throughout pregnancy lowers the risks of miscarriage, pre-eclampsia, and neurodevelopmental problems.
Synthroid is not a weight-loss medicine and should not be taken for obesity in people with normal thyroid function. Using thyroid hormone to speed metabolism without a medical indication is hazardous and may increase the risk of heart rhythm disturbances, bone loss, and other adverse outcomes.
Dosage and direction for taking Synthroid
Levothyroxine dosing is individualised. Prescribers consider body weight, age, cardiovascular history, and the cause of hypothyroidism to choose a starting dose, then fine-tune based on TSH and free T4 results. Many younger, otherwise healthy adults with clear hypothyroidism ultimately require about 1.6 micrograms per kilogram per day. Older adults and people with heart disease generally commence on lower doses and titrate slowly every 6–8 weeks to reduce the chance of cardiac symptoms.
Take Synthroid once daily on an empty stomach with water—ideally first thing in the morning. Wait 30–60 minutes before breakfast or coffee. Some people prefer a consistent bedtime dose taken at least 3–4 hours after the last meal. Whatever you choose, be consistent with timing and conditions to limit absorption variability and maintain stable thyroid levels.
Separate Synthroid from products that impair absorption: calcium and iron supplements, magnesium, multivitamins, aluminium- or calcium-containing antacids, sucralfate, bile acid sequestrants (cholestyramine, colestipol), sevelamer, and orlistat. Aim to space these by at least 4 hours. Share a complete medication and supplement list with your clinician or pharmacist to design a workable schedule.
Try to stay with the same brand where possible. Small potency differences between levothyroxine products can matter for sensitive patients. If you change brands or generic manufacturers, arrange a TSH check in 6–8 weeks. Likewise, repeat bloods after notable dietary shifts (for example, a large increase in soy or fibre), new medicines, or changes in adherence.
Children require carefully weight-based dosing that evolves as they grow. Infants with congenital hypothyroidism need prompt treatment to support normal brain development, with close follow-up. During pregnancy, most patients need a dose increase—sometimes as soon as there is a positive test—and TSH checks every 4–6 weeks. Postpartum, doses commonly return toward pre-pregnancy settings with follow-up pathology.
Do not double doses or self-adjust without guidance. If your TSH is outside target or you have symptoms of over- or under-treatment, contact your healthcare provider to plan a safe adjustment and repeat testing.
Precautions before and during Synthroid therapy
Cardiac health is a key consideration. Thyroid hormone increases heart rate and myocardial contractility. People with coronary artery disease, atrial fibrillation, or heart failure typically start on lower doses and titrate gradually to avoid angina or arrhythmias. Report palpitations, chest pain, dizziness, or breathlessness promptly.
Unrecognised adrenal insufficiency should be treated before starting levothyroxine. Thyroid hormone can accelerate cortisol metabolism and precipitate adrenal crisis when cortisol is low. If there are suggestive features—such as hyperpigmentation, salt craving, or low blood pressure—your clinician may assess adrenal function first.
Metabolic conditions can shift with treatment. As hypothyroidism resolves, requirements for insulin or oral hypoglycaemics may change, and anticoagulants like warfarin can have an enhanced effect as thyroid hormone increases. Closer monitoring around dose changes helps keep glucose and INR within target ranges.
Other cautions include osteoporosis risk (especially if long-term TSH suppression is used), seizure disorders, and malabsorption syndromes (such as coeliac disease or inflammatory bowel disease) that can alter levothyroxine needs. High-dose biotin can interfere with thyroid tests; pause biotin for 2–3 days before blood draws, or follow your pathology provider’s instructions.
Contraindications to Synthroid
Do not use Synthroid in uncorrected adrenal insufficiency or untreated thyrotoxicosis (hyperthyroidism). Hypersensitivity to any component of the tablet is a contraindication, though true allergy to levothyroxine itself is very rare.
Use levothyroxine cautiously and under close supervision in people with recent myocardial infarction, active cardiac ischaemia, or significant arrhythmias. If replacement is essential, start very low and titrate carefully.
Possible side effects of Synthroid
Most unwanted effects reflect too much or too little thyroid hormone. Signs of excess include nervousness, anxiety, insomnia, tremor, heat intolerance, sweating, loose stools, unintended weight loss, hair thinning, menstrual changes, palpitations, or chest discomfort. Contact your clinician if these occur—your dose may need adjustment.
Insufficient dosing resembles hypothyroidism: fatigue, cold intolerance, constipation, dry skin, hair loss, low mood, slowed thinking, and weight gain. Work with your provider to check adherence and timing relative to meals or supplements, and whether a gradual dose increase is appropriate.
Rare events include atrial fibrillation (particularly in older adults or those with cardiac disease), reduced bone mineral density with long-term over-replacement or deliberate TSH suppression, and in children, pseudotumour cerebri (headache, visual disturbances). Severe allergic reactions to excipients are uncommon but possible; seek urgent care for hives, swelling, or breathing difficulty.
If you are pregnant or planning pregnancy, inform your clinician early. Correct dosing lowers maternal and fetal risks; both under- and over-treatment can have consequences. Breastfeeding is generally compatible with Synthroid, and doses are adjusted 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, aluminium-containing antacids, sucralfate, bile acid sequestrants (cholestyramine, colestipol), sevelamer, lanthanum, and orlistat. High-fibre diets, soy products, and coffee taken near your dose can also reduce absorption; consistency helps.
Enzyme inducers can increase levothyroxine requirements by speeding T4 metabolism: carbamazepine, phenytoin, phenobarbital, rifampicin, and some tyrosine kinase inhibitors. If you start or stop these medicines, recheck TSH in 6–8 weeks.
Changes in protein and hormone binding affect dose needs. Oestrogen therapy (including pregnancy and combined oral contraceptives) raises thyroxine-binding globulin and often necessitates a higher Synthroid dose. Androgens can lower binding. Liver disease and nephrotic syndrome may also alter binding dynamics.
Clinically relevant pharmacodynamic interactions include potentiation of warfarin’s anticoagulant effect; monitor INR closely around levothyroxine dose changes. Serum digoxin levels may fall as thyroid status normalises, and insulin or oral hypoglycaemic requirements can shift. Sertraline and some SSRIs may increase thyroid hormone requirements, and amiodarone can disrupt thyroid function due to its iodine content.
High-dose biotin frequently interferes with lab assays for TSH and free T4. Pause biotin before testing per the guidance of your pathology service. Always share a complete list of medicines and supplements with your healthcare team and pharmacist.
Missed dose instructions for Synthroid
If you miss a dose, take it as soon as you remember that same day. If it is close to the time of your next dose, skip the missed tablet—do not take a double dose. Because levothyroxine has a long half-life, a single missed dose rarely causes issues. Keep your usual timing relative to meals and interacting supplements, and note the miss if symptoms reappear.
Overdose: signs and what to do
Excess Synthroid can trigger symptoms of hyperthyroidism: nervousness, tremor, rapid or irregular heartbeat, chest pain, shortness of breath, heat intolerance, diarrhoea, sweating, and insomnia. Severe overdose can lead to arrhythmias or, rarely, a thyroid storm–like presentation.
If you suspect an overdose—especially with chest pain, severe palpitations, confusion, or fainting—seek emergency care or call 000 (Triple Zero). For non-urgent advice, contact your clinician or the Poisons Information Centre on 13 11 26. Do not abruptly stop or self-correct dosing without medical advice unless instructed.
Storage recommendations
Store Synthroid at room temperature, protected from heat, light, and moisture. Avoid bathrooms and hot cars, which can degrade potency. Keep tablets in the original container with the desiccant, tightly closed, and out of reach of children and pets. Do not use beyond the expiry date; consistent potency is essential for stable thyroid control.
Australian sale and prescription policy for Synthroid
In Australia, Synthroid (levothyroxine) is a Schedule 4 (Prescription Only) medicine regulated by the Therapeutic Goods Administration (TGA). Buying Synthroid without a prescription is unsafe and may be unlawful. Safe treatment requires assessment by an Australian-registered prescriber, appropriate blood tests, individualised dosing, and periodic monitoring to avoid cardiac or bone complications from over- or under-treatment.
For convenience, Australians can access GP appointments in person or via Medicare-supported telehealth. Your clinician can order TSH and free T4 through accredited pathology services and issue an ePrescription (including via an Active Script List) for dispensing at community pharmacies. These legitimate pathways provide timely access while meeting safety and regulatory requirements.
Geisinger HealthSouth supports a compliant care pathway: connect with licensed clinicians for evaluation, obtain required pathology, and, when indicated, receive a valid prescription with coordinated pharmacy supply and follow-up. This protects you from the risks of no-prescription sales and ensures your dose is tailored, interactions are reviewed, and your therapy remains safe and effective over time. Where applicable, your clinician can also advise on potential PBS coverage and cost considerations in Australia.
