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Thyroid medication guide: levothyroxine strengths, timing and interactions
Levothyroxine is simple to take but easy to get slightly wrong. Learn which strength does what, the best way to take it, what interferes with it, and when to have your levels checked.
Updated October 2026 by the MyRxBox team
At a glance
- Active ingredientLevothyroxine (T4)
- Strengths25, 50, 75, 88, 100, 125 mcg
- Take itEmpty stomach, same time daily
- Blood test6–8 weeks after a dose change
What your thyroid does and what goes wrong
Your thyroid is a small gland in the neck that makes hormones controlling how fast your body uses energy. They affect heart rate, temperature, weight, mood, digestion, skin, hair and periods. When the gland makes too little hormone, the condition is hypothyroidism; too much is hyperthyroidism.
Hypothyroidism is far more common. The leading cause in many countries is Hashimoto’s thyroiditis, an autoimmune condition in which the immune system gradually damages the gland. Other causes include thyroid surgery, radioactive iodine treatment, some medicines and, worldwide, iodine deficiency.
Common symptoms of an underactive thyroid
- Tiredness and slowed thinking
- Feeling cold, dry skin and thinning hair
- Weight gain and constipation
- Low mood, muscle aches and heavier periods
These symptoms overlap with many other conditions, so diagnosis relies on blood tests, mainly TSH (thyroid-stimulating hormone) and free T4. For the full range of treatments, see our thyroid medications list.
Levothyroxine: the standard treatment
Levothyroxine is a synthetic version of T4, the main hormone your thyroid makes. It replaces what your gland can no longer produce. For most people with hypothyroidism it’s a lifelong, once-daily tablet, and one of the most prescribed medicines in the world. Eltroxin is a long-established levothyroxine brand.
| Strength | Typical use | Product |
|---|---|---|
| 25 mcg | Starting dose for older adults or people with heart disease; small adjustments | Thyroxine 25 mcg (Eltroxin) |
| 50 mcg | A common starting and adjustment dose | Thyroxine 50 mcg (Eltroxin) |
| 75 mcg | Maintenance for many adults | Thyroxine 75 mcg (Eltroxin) |
| 88 mcg | Fine-tuning between 75 and 100 mcg | Thyroxine 88 mcg (Eltroxin) |
| 100 mcg | Maintenance for many adults | Thyroxine 100 mcg (Eltroxin) |
| 125 mcg | Higher maintenance doses | Thyroxine 125 mcg (Eltroxin) |
Doses are based on body weight, age, heart health and blood test results. Many adults end up somewhere between 50 and 150 mcg a day, and some combine strengths to reach their exact dose. Only change your dose on your doctor’s advice.
How to take levothyroxine for steady results
Levothyroxine is easy to take but sensitive to timing, because food and some medicines reduce how much is absorbed. Consistency matters more than the exact time you choose.
- Take it at the same time every day, usually on waking.
- Take it on an empty stomach with a glass of water, 30 to 60 minutes before breakfast or coffee.
- Alternatively, some people take it at bedtime, at least 3 hours after their last meal. Pick one routine and stick to it.
- Separate it from interacting products by about 4 hours (see below).
- If you miss a dose, take it when you remember the same day. Don’t double up the next day unless your doctor advises it.
- Expect to feel better gradually, over several weeks.
What interferes with levothyroxine
| Product | Effect | What to do |
|---|---|---|
| Calcium and iron supplements, including ferrous iron | Bind levothyroxine in the gut and reduce absorption | Take at least 4 hours apart |
| Antacids, magnesium, aluminium | Reduce absorption | Take at least 4 hours apart |
| Bile acid binders such as colesevelam and colestipol | Strongly reduce absorption | Separate doses as your doctor advises |
| Acid reducers such as omeprazole, esomeprazole and pantoprazole | May reduce absorption over time | Your doctor may check your levels |
| Coffee, soy and high-fibre breakfasts | Reduce absorption if taken too close together | Wait 30 to 60 minutes after your tablet |
| Biotin supplements | Can distort thyroid blood test results | Stop a few days before blood tests, if your doctor agrees |
| Blood thinners such as warfarin | Thyroid dose changes can alter their effect | Extra monitoring after dose changes. See blood thinners guide |
Starting or stopping oestrogen-containing medicines, some anti-seizure medicines and certain diabetes treatments can also change your dose needs. Always share a full medicine list with your doctor; our A–Z of active ingredients helps you write one.
Dose adjustments and blood tests
Levothyroxine has a long half-life, so a dose change takes about six weeks to show its full effect on your blood tests. That’s why doctors usually recheck TSH 6 to 8 weeks after starting or changing a dose, then once a year when you’re stable.
Signs your dose may be too low are the original symptoms returning. Signs it may be too high include a racing or irregular heartbeat, anxiety, tremor, sweating, poor sleep and weight loss. Long-term over-replacement can thin bones and strain the heart, especially in older adults, so don’t adjust your dose based on symptoms alone.
Read our detailed guide: levothyroxine dosing and monitoring: why your dose changes, and what affects it.
Switching between levothyroxine brands
Levothyroxine has a narrow therapeutic range: small differences in absorption between brands can shift your levels. Switching is fine for most people, but your doctor may want a TSH check about 6 to 8 weeks afterwards. Once you’ve found a brand that works, staying with it keeps things simple.
Generic medicines contain the same active ingredient, but inactive ingredients differ. Read brand vs generic medicines to understand why this matters more for levothyroxine than for most medicines.
Pregnancy, older age and an overactive thyroid
Pregnancy
Thyroid hormone needs usually rise early in pregnancy, and well-controlled levels are important for the baby’s development. If you take levothyroxine and become pregnant, contact your doctor straight away; many people need a dose increase and more frequent blood tests. Levothyroxine is considered safe in pregnancy and breastfeeding.
Older adults and heart disease
Doctors usually start with a low dose, such as 25 mcg, and increase slowly to avoid straining the heart.
Overactive thyroid (hyperthyroidism)
An overactive thyroid needs different treatment, such as antithyroid medicines like methimazole or propylthiouracil, beta-blockers such as propranolol for symptoms, radioactive iodine or surgery. Levothyroxine is not a treatment for an overactive thyroid.
Ordering levothyroxine from MyRxBox
- Check your current prescription for the exact strength in micrograms (mcg).
- Choose the matching thyroid care product and a pack that covers your needs plus delivery time.
- Upload your prescription through our secure form; we check it before dispatch.
- Reorder early. Running out of levothyroxine isn’t an emergency for a day or two, but gaps should be avoided. See how to refill a prescription.
Country rules differ: see our USA, UK and Australia guides, and compare prices in the medicine price index.
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Levothyroxine questions, answered
Timing, food, brands and blood tests.
Still have a question? Contact our team.
What's the best time to take levothyroxine?
On waking, on an empty stomach with water, 30 to 60 minutes before breakfast or coffee. Bedtime dosing, 3 hours after food, also works if it suits you better. Consistency matters most.
Can I drink coffee after taking levothyroxine?
Wait at least 30 to 60 minutes. Coffee taken too soon can reduce absorption.
How long does levothyroxine take to work?
Many people feel better within a few weeks, but it takes about six weeks for blood levels to settle after a dose change.
Is Eltroxin the same as other levothyroxine brands?
They contain the same active ingredient, but small absorption differences mean your doctor may check your TSH after switching. See switching brands.
What if I miss a dose?
Take it when you remember that day. If you don’t remember until the next day, don’t double up unless your doctor tells you to.
Can I take levothyroxine with calcium or iron?
Take them at least 4 hours apart, as they reduce absorption.
Is levothyroxine safe in pregnancy?
Yes, and it’s important. Contact your doctor as soon as you know you’re pregnant, as most people need a dose increase.
Will levothyroxine help me lose weight?
Correcting an underactive thyroid can help with weight gained because of it, but levothyroxine shouldn’t be used for weight loss. Excess thyroid hormone harms the heart and bones.
Never run short of your thyroid medicine
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