Starting Dose for Levothyroxine
For adults under 70 years without cardiac disease, start levothyroxine at 1.6 mcg/kg/day based on ideal body weight; for patients over 70 years or with cardiac disease, start at 25-50 mcg/day. 1, 2, 3
Age and Comorbidity-Based Dosing Algorithm
Younger Adults Without Cardiac Disease (<70 years)
Full replacement dosing of 1.6 mcg/kg/day is safe and appropriate for cardiac asymptomatic patients with primary hypothyroidism, typically translating to 75-100 mcg/day for women and 100-150 mcg/day for men. 1, 2, 3, 4
A prospective randomized trial demonstrated that full-dose initiation (1.6 mcg/kg) is safe with no cardiac events documented, and achieves euthyroidism significantly faster than low-dose titration (13 vs 1 patient euthyroid at 4 weeks, p=0.005). 5
This approach is more convenient and cost-effective than gradual titration, as patients reach target TSH levels in 4-8 weeks rather than 20-24 weeks. 5
Older Adults and High-Risk Patients (≥70 years or cardiac disease)
Start with 25-50 mcg/day and titrate gradually to avoid exacerbating cardiac symptoms, particularly angina or atrial fibrillation. 1, 2, 3
Elderly patients require approximately one-third less levothyroxine than younger populations—mean euthyroid dose is 1.09 mcg/kg actual body weight (or 1.35 mcg/kg ideal body weight) compared to 1.6 mcg/kg in younger adults. 6, 7
For patients at risk of atrial fibrillation or with underlying cardiac disease, titrate dosage every 6-8 weeks rather than every 4-6 weeks to minimize cardiac complications. 3
Special Population Considerations
Pregnant Patients
For new-onset hypothyroidism with TSH ≥10 mIU/L, start at 1.6 mcg/kg/day; for TSH <10 mIU/L, start at 1.0 mcg/kg/day. 2, 3
Pre-existing hypothyroidism requires immediate dose increase by 25-50% upon pregnancy confirmation, as levothyroxine requirements increase 25-50% during early pregnancy. 1, 2
Monitor TSH every 4 weeks during pregnancy and adjust to maintain TSH within trimester-specific reference ranges. 2, 3
Subclinical Hypothyroidism
For TSH >10 mIU/L with normal free T4, initiate levothyroxine at standard age-appropriate doses regardless of symptoms, as this threshold carries approximately 5% annual risk of progression to overt hypothyroidism. 1
For TSH 4.5-10 mIU/L, treatment decisions should be individualized based on symptoms, positive anti-TPO antibodies (which increase progression risk to 4.3% per year), or pregnancy planning. 1
Obese Patients
- Calculate dosing using ideal body weight rather than actual body weight, as obese individuals require similar doses per kilogram ideal body weight (1.42 vs 1.32 mcg/kg IBW) but significantly lower doses per actual body weight (0.9 vs 1.14 mcg/kg). 6
Dose Titration Protocol
Monitor TSH every 6-8 weeks during dose titration, adjusting by 12.5-25 mcg increments until TSH normalizes to 0.5-4.5 mIU/L. 1, 2, 3
Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize than free T4. 1, 2
Once adequately treated with stable TSH, repeat testing every 6-12 months or when symptoms change. 1, 2
Critical Safety Considerations
Before Initiating Therapy
Always rule out concurrent adrenal insufficiency before starting levothyroxine, as thyroid hormone replacement can precipitate life-threatening adrenal crisis in patients with unrecognized hypopituitarism or primary adrenal insufficiency. 1
In suspected central hypothyroidism or hypophysitis, start physiologic-dose corticosteroids 1 week prior to levothyroxine initiation. 1
Monitoring for Overtreatment
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications. 1
Prolonged TSH suppression (<0.1 mIU/L) significantly increases risk of atrial fibrillation (5-fold in patients ≥45 years) and fractures, particularly in postmenopausal women. 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, particularly in recovery phase thyroiditis or nonthyroidal illness. 1
Avoid starting with full replacement doses in elderly patients or those with cardiac disease, as this can unmask or worsen cardiac ischemia, precipitate angina, or trigger atrial fibrillation. 1, 3
Never assume hypothyroidism is permanent without reassessment—consider transient thyroiditis, particularly in patients on immune checkpoint inhibitors where thyroid dysfunction occurs in 6-20% but may be temporary. 1
For patients with severe hypothyroidism (TSH >50-100 mIU/L) or myxedema, hospital admission with endocrinology consultation is required rather than outpatient initiation. 2