What is the typical treatment and dosage for hypothyroidism using Synthroid (levothyroxine)?

Synthroid (Levothyroxine) Treatment for Hypothyroidism

Initial Dosing Strategy

For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day (typically 75-100 mcg for women, 100-150 mcg for men), which rapidly normalizes thyroid function and prevents complications of untreated hypothyroidism. 1, 2

• For patients over 70 years or those with cardiac disease, atrial fibrillation, or coronary artery disease, start with a lower dose of 25-50 mcg/day to avoid exacerbating cardiac symptoms 1, 2, 3
• Elderly patients with underlying coronary disease are at increased risk of cardiac decompensation, angina, or arrhythmias even with therapeutic levothyroxine doses 1

Dose Titration and Monitoring

Monitor TSH every 6-8 weeks after initiating therapy or changing doses, then annually once TSH is within the target range of 0.5-4.5 mIU/L. 1, 2, 4

• Adjust levothyroxine by 12.5-25 mcg increments every 4-6 weeks based on TSH and free T4 levels until the patient is euthyroid 1, 2
• Use smaller increments (12.5 mcg) for elderly patients or those with cardiac disease to minimize cardiac complications 1
• The peak therapeutic effect of a given dose may not be attained for 4-6 weeks 2
• Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize 1

Treatment Indications Based on TSH Levels

Initiate levothyroxine therapy for all patients with TSH >10 mIU/L regardless of symptoms, as this level carries approximately 5% annual risk of progression to overt hypothyroidism. 1, 4

• For TSH 4.5-10 mIU/L with normal free T4 (subclinical hypothyroidism), treatment decisions should be individualized based on symptoms, pregnancy status, or positive anti-TPO antibodies 1, 3
• Symptomatic patients with fatigue, weight gain, cold intolerance, or constipation may benefit from a 3-4 month trial of levothyroxine even with TSH 4.5-10 mIU/L 1
• Patients with positive anti-TPO antibodies have a 4.3% annual progression risk to overt hypothyroidism versus 2.6% in antibody-negative individuals 1

Special Populations

Pregnant Patients

Women with pre-existing hypothyroidism should increase their levothyroxine dose by 25-50% (approximately 30% weekly dosage increase, taking one extra dose twice per week) as soon as pregnancy is confirmed. 1, 2, 3

• Monitor TSH every 4 weeks during pregnancy and maintain TSH in the trimester-specific reference range 1, 2
• Reduce levothyroxine to pre-pregnancy levels immediately after delivery and monitor TSH 4-8 weeks postpartum 2
• Subclinical hypothyroidism during pregnancy is associated with preeclampsia, low birth weight, and potential neurodevelopmental effects in offspring 1

Patients on Immunotherapy

For patients on immune checkpoint inhibitors (anti-PD-1/PD-L1 therapy) with thyroid dysfunction, consider treatment even for subclinical hypothyroidism if fatigue or other symptoms are present. 1

• Thyroid dysfunction occurs in 6-9% with anti-PD-1/PD-L1 therapy and 16-20% with combination immunotherapy 1
• Continue immunotherapy in most cases, as thyroid dysfunction rarely requires treatment interruption 1
• Monitor TSH every cycle for the first 3 months, then every second cycle thereafter 1

Critical Safety Considerations

Before initiating levothyroxine in patients with suspected central hypothyroidism or hypophysitis, always start corticosteroids first to prevent precipitating adrenal crisis. 1

• In patients with concurrent adrenal insufficiency, initiate physiologic dose steroids 1 week prior to thyroid hormone replacement 1
• Never start thyroid hormone before ruling out adrenal insufficiency in patients with suspected pituitary or hypothalamic dysfunction 1

Risks of Overtreatment

Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, significantly increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications. 1, 4

• Prolonged TSH suppression (<0.1 mIU/L) increases risk for atrial fibrillation, especially in elderly patients 1
• Overtreatment increases risk for osteoporotic fractures, particularly in postmenopausal women 1
• TSH suppression is associated with left ventricular hypertrophy, abnormal cardiac output, and potential increased cardiovascular mortality 1
• If TSH becomes suppressed during therapy, reduce levothyroxine dose by 12.5-25 mcg to allow TSH to increase toward the reference range 1

Common Pitfalls to Avoid

• Do not treat based on a single elevated TSH value, as 30-60% of elevated TSH levels normalize spontaneously on repeat testing 1
• Avoid adjusting doses too frequently before reaching steady state—wait 6-8 weeks between adjustments 1, 2
• Do not assume hypothyroidism is permanent without reassessment—consider transient thyroiditis, especially in recovery phase 1
• Avoid excessive dose increases that could lead to iatrogenic hyperthyroidism, which occurs in 14-21% of treated patients 1

Confirmation Testing

Confirm elevated TSH with repeat testing after 3-6 weeks along with free T4 measurement before initiating treatment. 1

• Measure both TSH and free T4 to distinguish subclinical hypothyroidism (normal free T4) from overt hypothyroidism (low free T4) 1
• Consider measuring anti-TPO antibodies to confirm autoimmune etiology and predict progression risk 1
• For patients with cardiac disease or atrial fibrillation, consider repeating testing within 2 weeks rather than waiting the full 3-6 weeks 1