Synthroid. What diseases does it treat?

Take Synthroid (levothyroxine) once daily on an empty stomach, 30–60 minutes before breakfast. Typical initial dosing: healthy adults ≈1.6 mcg/kg/day (most fall between 75–125 mcg/day); patients >60 years or with ischemic heart disease start 12.5–25 mcg/day and increase slowly; severe hypothyroidism or myxedema coma often requires an IV bolus of 200–500 mcg followed by 50–100 mcg IV daily. Reassess dose with labs every 6–8 weeks until levels stabilize.

Prescribe levothyroxine for primary hypothyroidism, hormonal replacement after thyroidectomy or radioiodine, suppression therapy for differentiated thyroid cancer (suppression dose depends on recurrence risk and may exceed 150 mcg/day), symptomatic subclinical hypothyroidism when TSH >10 mIU/L or symptoms impair daily function, and selected cases of goiter. In autoimmune thyroiditis (Hashimoto’s), start lifelong replacement when elevated TSH and low free T4 confirm deficiency.

Monitor treatment with TSH and free T4: test 6–8 weeks after starting or adjusting dose, then every 6–12 months once stable. Aim for TSH 0.5–2.5 mIU/L in most adults; for patients aged >65 or with coronary disease choose a higher, individualized target (commonly 4.0–6.0 mIU/L). For high-risk thyroid cancer maintain TSH <0.1 mIU/L under specialist guidance.

Reduce absorption interference by separating levothyroxine from calcium, iron, magnesium, bile acid sequestrants and antacids; wait about 4 hours before taking those supplements after levothyroxine. Stick with the same manufacturer or formulation and repeat TSH testing 6–8 weeks after any brand switch. If you miss a dose, take it when remembered unless the next dose is imminent; do not double doses.

Use caution in recent myocardial infarction, untreated adrenal insufficiency and unstable arrhythmias: initiate low doses and titrate gradually to avoid cardiac stress. Pregnant patients usually require a dose increase of ~25–30% and need TSH checks each trimester. Coordinate dose changes and target selection with a clinician when comorbid conditions or pregnancy are present.

Dosage and titration for primary hypothyroidism: how to start and when to retest

Begin most nonelderly adults without cardiac disease on roughly 1.6 mcg/kg/day of levothyroxine (Synthroid) as full replacement; calculate dose from actual body weight for nonobese patients and from ideal or adjusted body weight for obese patients.

Practical titration schedule

Adjust dose in 12.5–25 mcg increments every 4–6 weeks based on TSH until the target range is reached. Recheck TSH 6–8 weeks after initiation or any dose change because TSH reflects steady-state thyroid status at that interval. For large deficits (TSH very high or symptomatic hypothyroidism) consider 25–50 mcg steps guided by clinical response and labs; reduce adjustment size in older adults or those with coronary disease.

Patient group	Typical starting dose	Titration and retest
Neonates (congenital hypothyroidism)	10–15 mcg/kg/day	Daily dosing; check TSH and free T4 within 1–2 weeks, then every 2–4 weeks until stable
Infants	6–8 mcg/kg/day	Labs every 2–4 weeks initially
Children (older)	4–6 mcg/kg/day (age-dependent)	Monitor every 4–6 weeks until stable
Healthy adult <50 years, no cardiac disease	~1.6 mcg/kg/day	TSH at 6–8 weeks; adjust by 12.5–25 mcg as needed
Adults ≥50 years or with coronary disease	Start 25–50 mcg/day (often 12.5–25 mcg if frail)	Increase slowly (12.5–25 mcg every 4–6 weeks); check TSH 6–8 weeks
Very elderly or frail	12.5–25 mcg/day	Small increments; monitor symptoms and TSH closely
Pregnancy	Increase preconception dose by ~25–30% once pregnancy is confirmed	Check TSH in 4 weeks after change, then every 4–6 weeks during first half of pregnancy, at least once in late second/third trimester
Severe hypothyroidism / myxedema	Low initial doses (12.5–25 mcg) with inpatient management as needed	Slow titration; frequent labs and clinical monitoring

Administration and monitoring notes

Take levothyroxine on an empty stomach 30–60 minutes before breakfast or at bedtime at least 3–4 hours after the last meal. Separate dosing from calcium, iron, antacids, multivitamins containing minerals, and sucralfate by at least 4 hours. If you switch brands or formulations, recheck TSH in 6–8 weeks and adjust dose if needed.

Aim for a TSH target appropriate to the patient: about 0.5–2.5 mIU/L in younger, symptomatic adults; accept a higher TSH (for example 4–6 mIU/L) in older patients or those with coronary disease to avoid overtreatment. Use free T4 (not TSH) to guide therapy in central hypothyroidism and follow clinical status as well as labs. Once stable, check TSH annually or sooner with new symptoms, pregnancy, interacting medications, or changes in absorption.

Managing Hashimoto’s thyroiditis on Synthroid: indicators for dose change and follow-up tests

Change levothyroxine dose when TSH or clinical status indicate under- or over-replacement, and confirm adjustments with timed laboratory testing.

• Primary laboratory targets

  • Use the laboratory reference range for TSH (commonly 0.4–4.0 mIU/L) as the baseline target.
  • For symptomatic patients or those seeking symptom control, target roughly 0.4–2.5 mIU/L.
  • Pregnancy goals: first trimester TSH <2.5 mIU/L; second and third trimester <3.0 mIU/L (adjust dose quickly and recheck often).
  • For older adults or patients with ischemic heart disease accept a higher TSH within the lab range to reduce cardiac risk; individualize based on symptoms and comorbidity.

• When to increase dose (under-replacement)

  • TSH above the chosen target range (example: TSH >4.0 mIU/L for most patients).
  • Persistent hypothyroid symptoms: fatigue, weight gain, cold intolerance, constipation, dry skin, elevated LDL cholesterol.
  • New pregnancy or planned conception (increase dose promptly and recheck TSH every 4 weeks initially).
  • New medications that reduce levothyroxine levels (enzyme inducers such as rifampin, phenytoin, carbamazepine; chronic cholestyramine; starting proton pump inhibitors in some cases).
  • Conditions reducing absorption: celiac disease, untreated H. pylori, recent bariatric surgery, or starting calcium/iron supplements without appropriate spacing.
  • Substantial weight gain (dose per kg may need upward adjustment).

• When to decrease dose (over-replacement)

  • TSH suppressed <0.1 mIU/L or trending downward beyond the lower target, especially with symptoms of hyperthyroidism.
  • New symptoms of excess thyroid hormone: palpitations, unexplained weight loss, increased bowel frequency, tremor, anxiety, atrial fibrillation.
  • Evidence of bone loss (postmenopausal women with prolonged TSH suppression) or new arrhythmia.
  • Starting interacting drugs that raise levothyroxine levels or increase sensitivity (for example, stopping enzyme inducers, initiating estrogen withdrawal).

• Recommended dose-adjustment steps and magnitudes

  • Typical incremental change: 12.5–25 mcg for most adults.
  • Older adults or patients with coronary artery disease: use 12.5 mcg steps and monitor closely for cardiac symptoms.
  • If TSH >10 mIU/L or severe hypothyroid signs, consider larger increases (25–50 mcg or full replacement calculated by weight) guided by clinical context.
  • When TSH <0.1 mIU/L with symptomatic hyperthyroidism, lower dose by 25–50 mcg depending on severity and re-evaluate.
  • After a single dose adjustment, wait for biochemical stabilization before further change unless the patient is pregnant or clinically unstable.

• Follow-up testing schedule

  • After any dose change: check TSH (and free T4 if indicated) 6–8 weeks later; this interval allows steady-state equilibration.
  • Large dose changes, pregnancy-related adjustments, or major interacting drug changes: recheck TSH at 4 weeks until stable, then every 4–8 weeks as needed.
  • After starting or stopping medications that affect absorption (calcium, iron, bile acid binders) test at 6–8 weeks.
  • Once dose and labs are stable: measure TSH every 6–12 months or sooner with new symptoms or health changes.
  • Draw TSH consistently relative to dosing (preferably before the morning dose or at the same time of day) to reduce variability.

• Additional tests and monitoring

  • Free T4: check alongside TSH if central hypothyroidism is suspected or if discordant results/symptoms occur.
  • Free T3: consider only when persistent symptoms exist despite normal TSH/free T4 or when conversion issues are suspected.
  • TPO antibodies: measure once for diagnostic confirmation and prognostic information; do not use antibody titer to adjust replacement dose.
  • Thyroid ultrasound: obtain if rapidly enlarging gland, new nodules, or focal pain.
  • Bone density screening and cardiac evaluation: perform for patients with prolonged TSH suppression, postmenopausal women, or those developing arrhythmias.

• Practical medication and lab tips

  • Take levothyroxine on an empty stomach 30–60 minutes before breakfast or at bedtime at least 3 hours after the last meal; maintain the same routine when having labs drawn.
  • Separate levothyroxine from calcium, iron, sucralfate and certain supplements by 4 hours to preserve absorption.
  • When switching brands or formulations, recheck TSH in 6–8 weeks and adjust dose if necessary.
  • Document weight, new medications, pregnancy status, and GI symptoms at each visit to identify causes of dose drift.

Coordinate dose changes with a clinician experienced in thyroid management and use objective TSH/free T4 results plus the clinical picture to guide safe, measured adjustments.

Post-thyroidectomy replacement with Synthroid: initial dosing and long-term monitoring plan

Start levothyroxine immediately after total thyroidectomy using a weight-based oral dose of 1.6 mcg/kg/day (ideal body weight) for healthy adults who tolerate full replacement. For patients >60 years or with known ischemic heart disease choose a lower initial dose (25–50 mcg/day); for very frail or unstable cardiac patients start 12.5–25 mcg/day and increase in small increments. After lobectomy, begin with 1.0–1.2 mcg/kg/day if postoperative hypothyroidism risk is high; if residual thyroid tissue is expected, use a lower empiric dose and plan earlier reassessment.

Measure TSH and free T4 at 6 weeks after starting therapy or after any dose change. Adjust dose by 12.5–25 mcg increments based primarily on TSH: aim for a replacement range of TSH ~0.5–2.0 mIU/L for benign disease. If treating thyroid cancer with suppression strategy, set targets by risk: low risk TSH 0.1–0.5 mIU/L, intermediate/high risk TSH <0.1 mIU/L; check TSH and free T4 every 6–8 weeks until the suppression target is reached, then every 3–6 months during the first year and every 6–12 months thereafter.

When converting from IV postoperative levothyroxine to oral therapy, transition as soon as the patient tolerates enteral intake using the calculated oral replacement dose (weight-based). Give Synthroid on an empty stomach 60 minutes before breakfast or at bedtime >3 hours after the last meal. Separate dosing from calcium, iron, antacids and multivitamins by at least 4 hours to avoid reduced absorption.

In pregnancy increase the oral levothyroxine dose by approximately 25–30% as soon as pregnancy is confirmed and check TSH every 4 weeks until values stabilize, then every 4–6 weeks through mid-pregnancy and at least once each trimester thereafter. For renal failure, advanced age, or other comorbidities lean toward lower initial doses and slower titration.

Expect biochemical response to a dose change after ~6 weeks. Watch for signs of over-replacement (palpitations, heat intolerance, unexplained weight loss, atrial arrhythmia) and under-replacement (fatigue, weight gain, cold intolerance, constipation). Expedite lab testing and cardiology review if new chest pain, new atrial fibrillation, or marked tremor occur after dose increases.

Synthroid during pregnancy and lactation: timing of dose adjustments and maternal–fetal monitoring

Increase a woman’s levothyroxine dose by about 25–30% as soon as pregnancy is confirmed; a practical method is to take two extra usual doses per week (for example, if taking 100 mcg daily, add 100 mcg on two additional days each week).

Measure TSH every 4 weeks until mid-pregnancy (through about 20 weeks), then at least once each trimester thereafter. Target TSH ranges: first trimester 0.1–2.5 mIU/L, second trimester 0.2–3.0 mIU/L, third trimester 0.3–3.0 mIU/L. Adjust dose if TSH lies outside these trimester-specific limits.

If a pregnant patient is newly diagnosed with overt hypothyroidism, start full replacement immediately and recheck TSH in 2–4 weeks. For subclinical hypothyroidism, treat if TSH exceeds the trimester-specific upper limit or if thyroid peroxidase antibodies (TPOAb) are positive; recheck TSH 4 weeks after any change.

Make incremental dose changes of 12.5–25 mcg for fine tuning; larger adjustments (20–30%) are reasonable for the initial pregnancy increase or for marked hypothyroidism. If TSH becomes suppressed (<0.1 mIU/L), reduce the dose by 25–50 mcg and re-evaluate in 4 weeks. For central (secondary) hypothyroidism monitor free T4 rather than TSH and target mid-to-upper reference-range free T4.

Escalate maternal–fetal surveillance if thyroid control remains poor despite dose increases, if TSH exceeds 10 mIU/L, or if overt hypothyroidism was present at conception. Recommended actions include referral to maternal–fetal medicine, targeted anatomy scan at 18–20 weeks if not already planned, and periodic fetal growth assessment later in pregnancy when clinical concern exists.

At delivery reduce the levothyroxine dose back to the pre-pregnancy amount unless laboratory results indicate otherwise; check TSH at about 6 weeks postpartum. Continue levothyroxine during lactation–breastfeeding does not require dose cessation. Separate levothyroxine from calcium- or iron-containing supplements by at least 4 hours and take levothyroxine on an empty stomach (30–60 minutes before breakfast or at bedtime at least 3 hours after the last meal).

Screen for postpartum thyroid dysfunction if new symptoms arise; measure TSH at 6 and 12 weeks postpartum in symptomatic women or those with positive TPOAb. Adjust therapy based on the same laboratory targets and retest 4–6 weeks after any dose change.

When to treat subclinical hypothyroidism with Synthroid: patient selection and expected clinical goals

Start levothyroxine (Synthroid) for patients with TSH ≥10 mIU/L; use replacement dosing adjusted for age and cardiac status and recheck TSH 6–8 weeks after initiation.

Treat patients with TSH 4.5–10 mIU/L when any of the following apply: symptomatic hypothyroid complaints (fatigue, cold intolerance, constipation, cognitive slowing) that correlate with abnormal free T4 or positive anti-thyroid peroxidase (TPO) antibodies; planning pregnancy or currently pregnant; infertility or recurrent miscarriage; new or progressive dyslipidemia (LDL elevation despite therapy) or atherosclerotic cardiovascular disease; enlarging goiter or compressive symptoms; or persistent TSH elevation on repeat testing at 3 months.

When selecting candidates, confirm TSH on at least one repeat measurement (same laboratory, fasting not required) and check free T4 and TPO antibodies. If TPO-positive, risk of progression to overt hypothyroidism rises ~3–5% per year; this supports treatment at lower TSH thresholds. If TPO-negative and asymptomatic, observe and remeasure TSH in 3–6 months before treating.

Initial dosing: for adults <65 without cardiac disease, use 25–50 mcg/day for TSH 4.5–10 and 1.0–1.6 mcg/kg/day for TSH ≥10 (round to practical tablet strengths). For patients ≥65, those with ischemic heart disease or arrhythmia, begin 12.5–25 mcg/day and increase slowly every 6–8 weeks based on TSH and symptoms. Adjust dose in low-body-weight or frail patients toward the lower end.

Aim TSH within the laboratory reference range (commonly 0.4–4.0 mIU/L). For patients trying to conceive or pregnant, aim for TSH <2.5 mIU/L preconception and in the first trimester; adjust per trimester-specific targets thereafter. Avoid TSH suppression <0.1 mIU/L unless indicated for thyroid cancer management.

Expect partial or full symptom improvement within 6–12 weeks after achieving target TSH; assess fatigue, weight trends, bowel habits and cognition during follow-up. Expect modest LDL reductions (5–15%) within 6–12 weeks if hypothyroidism contributed to dyslipidemia; continue lipid monitoring to gauge clinical benefit.

Monitoring plan: check TSH 6–8 weeks after any dose change, then every 6–12 months once stable. In elderly or cardiac patients, monitor earlier and increase dose increments more slowly. If TSH remains elevated on therapy, confirm adherence, review interfering medications and supplements (iron, calcium, bile acid sequestrants), and consider increasing dose in 12.5–25 mcg steps.

Stop or withhold dose increases when TSH falls below the target range or when new atrial arrhythmia or angina appears. If TSH <0.1 mIU/L, reduce dose by 12.5–25 mcg and recheck in 6–8 weeks to avoid iatrogenic thyrotoxicosis and bone loss.

If symptoms persist despite normalized TSH, evaluate alternative causes (anemia, sleep apnea, depression, vitamin D deficiency) before escalating levothyroxine. Document goals with the patient: TSH target, expected timeline for symptom change, planned follow-up interval and thresholds for dose adjustment.