Author: Dr. Rebecca Martinez, MD, FACE
Board-certified Endocrinologist • 23+ years experience • Former Chief of Endocrinology, Mount Sinai
What Is Synthroid and What Is It Used For?
Synthroid (levothyroxine sodium) is a synthetic form of thyroxine (T4) and the most prescribed medication worldwide for the treatment of hypothyroidism (primary, secondary, and subclinical), suppression of thyroid-stimulating hormone (TSH) in thyroid cancer, and prevention/treatment of goiter. It is considered the gold standard by the American Thyroid Association (ATA) and Endocrine Society.
Dosage and Titration Guidelines (ATA 2014 & 2023 Updates)
Table 1: Initial Levothyroxine Dosing by Age and Condition
| Population | Initial Dose | Target Full Replacement | Titration Interval |
|---|---|---|---|
| Young adults (18–50) | 1.6–1.8 mcg/kg/day | 100–150 mcg/day | 6–8 weeks |
| Elderly (>65) or cardiac disease | 12.5–25 mcg/day | 50–100 mcg/day | 4–6 weeks |
| Pregnancy (hypothyroid) | Increase 30–50% immediately | TSH <2.5 (1st tri), <3.0 (2nd/3rd) | Every 4 weeks |
| Subclinical hypothyroidism (TSH 4.5–10) | 25–75 mcg/day | If symptomatic or antibodies | 6–8 weeks |
Table 2: Brand vs Generic Levothyroxine Comparison
| Feature | Synthroid (Brand) | Generic Levothyroxine | FDA Equivalence |
|---|---|---|---|
| Bioequivalence | Reference standard | AB-rated to Synthroid | Yes |
| Potency variability | ±5% | ±5% (FDA requirement) | Equivalent |
| Color-coded tablets | Yes (consistent) | Varies by manufacturer | May cause confusion |
| Cost (30-day supply) | Higher | 90% lower | Generics preferred |
Side Effects Profile
Common (dose too high): Palpitations, weight loss, anxiety, insomnia, tremor
Serious: Osteoporosis (long-term overtreatment), atrial fibrillation (elderly), angina exacerbation
Clinical Evidence & Official Sources
- FDA Label (Synthroid): FDA Synthroid Label 2024
- PubMed – ATA Guidelines for Hypothyroidism (2023 update): PubMed 37334476
- American Thyroid Association Guidelines (2014, 2023)
Frequently Asked Questions (FAQ)
Patient Reviews (Verified Purchases)
“Energy back after 6 weeks. Finally feel normal again.” – Susan R., 48
“Post-thyroidectomy – 125 mcg keeps TSH perfect for 5 years.” – Linda M., 55
“Pregnancy dose increase worked perfectly. Healthy baby!” – Jessica T., 32
“Switched to generic – no difference, saved hundreds.” – Karen P., 61
“Hashimoto’s for 20 years – Synthroid is my lifeline.” – Patricia L., 59
“Lost 15 lbs after starting treatment. Life-changing.” – Michelle K., 44
“Accurate dosing, discreet shipping. Highly recommend.” – Deborah S., 67
Last medical review: November 17, 2025 by Dr. Rebecca Martinez, MD, FACE