Levothyroxine is the standard treatment for an underactive thyroid (hypothyroidism) — replacing the hormone the thyroid cannot make sufficiently. Correct dosing and timing maximises effectiveness.
Levothyroxine (L-thyroxine, T4) is a synthetic form of thyroxine — the hormone produced by the thyroid gland — used to treat hypothyroidism (underactive thyroid) and other conditions including goitre and thyroid cancer follow-up. Hypothyroidism affects millions of people globally, causing fatigue, weight gain, cold intolerance, constipation, depression, and dry skin. Once started, levothyroxine treatment is usually lifelong and transforms quality of life when dosed correctly.
In Korea, hypothyroidism is diagnosed through a blood test measuring TSH (thyroid-stimulating hormone) and free T4. TSH is the most sensitive indicator of thyroid function — elevated TSH with normal or low free T4 confirms hypothyroidism. Treatment is initiated by GPs at local clinics and endocrinologists, with TSH monitoring every 6–12 weeks after starting or dose changes, then annually once stable. The target TSH for most patients is 0.5–2.5 mIU/L.
Levothyroxine must be taken correctly to achieve optimal absorption. The standard recommendation is to take it on an empty stomach, 30–60 minutes before breakfast, with water only — coffee, milk, food, calcium supplements, antacids, and iron tablets all significantly reduce absorption and should be taken at least 4 hours apart. Evening dosing (taking levothyroxine before bed, at least 3–4 hours after the last meal) is an evidence-based alternative that some patients prefer for convenience and may actually improve TSH levels. Many patients in Korea use this approach to avoid complex morning routines.
Key Points
- Take on empty stomach, 30–60 minutes before breakfast — OR before bed
- Coffee, calcium, iron, and antacids reduce absorption — take 4h apart
- TSH monitored every 6–12 weeks after starting or dose changes
- Do not change brands without doctor's guidance — potency may vary slightly
- Treatment is usually lifelong — do not stop without medical advice
- Symptoms improve gradually over 4–8 weeks after correct dose achieved
When to See a Doctor
- Symptoms not improving after 8 weeks on stable levothyroxine dose
- Palpitations, anxiety, or weight loss (possible over-treatment / high T4)
- TSH check due (first check at 6–12 weeks, then annually)
- Starting or stopping calcium supplements, iron, or antacids
- Pregnancy — levothyroxine requirements increase during pregnancy
For emergencies in Korea, call 119
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This content is published for educational purposes only. For accurate diagnosis and personalised treatment, consult a licensed KMA physician in Korea.
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Medical Disclaimer
This article is published by DoctorsInKorea.com for educational and informational purposes only. It does not replace a professional clinical evaluation by a licensed healthcare provider. For accurate medical diagnosis and personalised treatment paths, please book an appointment with a licensed KMA physician. For immediate life-threatening medical emergencies in Korea, please call 119.
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Last reviewed: March 1, 2025