Levothyroxine is among the most prescribed medicines in the world. In the United States it has topped or near-topped the prescription volume tables for over a decade. Tens of millions of people take it daily, most of them for life.
It is also, for such a widely used drug, remarkably easy to take incorrectly — not through carelessness, but because the instructions that matter most are the ones least often explained. The tablet is simple. The absorption is not.
This guide covers what levothyroxine is, how prescribers arrive at a dose, why that dose changes over the years, the interactions that quietly undermine it, and what to do when you feel unwell despite blood tests that look fine.
One thing to be clear about at the outset: levothyroxine dosing is driven by blood tests, not by symptoms and not by how you feel on a given week. Every number in this article describes what a prescriber works with when they have your TSH result in front of them. None of it is a basis for adjusting your own dose. The reasons for that will become obvious by the end.
What the thyroid does, and what goes wrong
The thyroid gland sits at the front of the neck and produces two hormones: thyroxine (T4) and triiodothyronine (T3). T4 is produced in far greater quantity but is relatively inactive. T3 is the metabolically active form. Most T3 in the body is not made by the thyroid at all — it’s converted from T4 in peripheral tissues, principally liver and kidney, by enzymes called deiodinases.
Together these hormones set the metabolic tempo of essentially every tissue. Heart rate and cardiac output. Gut motility. Body temperature. Cognitive speed. Skin and hair turnover. Menstrual regularity. Bone remodelling. Cholesterol clearance.
The system is controlled by a feedback loop. The pituitary gland monitors circulating thyroid hormone and releases thyroid stimulating hormone (TSH) to push the thyroid harder when levels drop. This relationship is inverse and it is the single most important concept for interpreting your blood results:
When thyroid hormone is low, TSH goes up. When thyroid hormone is high, TSH goes down.
So a high TSH means under-treatment. A low TSH means over-treatment. This trips up almost everyone the first time they see their results.
The relationship is also log-linear rather than proportional — small changes in free T4 produce large swings in TSH. That’s what makes TSH such a sensitive marker, and it’s why a result can look dramatic when the underlying hormone change is modest.
Hypothyroidism is underproduction. Globally the commonest cause is iodine deficiency; in iodine-replete countries it’s Hashimoto’s thyroiditis, an autoimmune condition in which the immune system gradually destroys thyroid tissue. Other causes include thyroid surgery, radioiodine treatment, certain drugs — amiodarone and lithium notably — and congenital absence.
Symptoms develop slowly and are easy to attribute to ordinary life: fatigue, cold intolerance, weight gain, constipation, dry skin, hair thinning, heavy or irregular periods, low mood, slowed thinking, muscle aches. Because onset is gradual, many people don’t recognise how unwell they’d become until treatment starts working.
What levothyroxine is
Levothyroxine is synthetic T4, chemically identical to what the thyroid produces. It’s stocked here as Eltroxin across six strengths: 25mcg, 50mcg, 75mcg, 88mcg, 100mcg and 125mcg.
A reasonable question is why the drug is T4 rather than the active T3. Two reasons. First, T4 has a half-life of about seven days, so once-daily dosing produces stable blood levels with little peak-to-trough variation. T3’s half-life is under a day, producing sharp peaks that the body never normally experiences. Second, giving T4 lets the body’s own deiodinase enzymes convert to T3 at whatever rate each tissue requires — you’re supplying the substrate and letting the existing regulatory machinery do the rest.
That long half-life explains two practical facts you’ll meet repeatedly: missing a single dose barely moves your levels, and any dose change takes about six weeks to fully show up in blood tests.
How a starting dose is chosen
For a healthy adult with no cardiac history, full replacement is roughly 1.6 micrograms per kilogram of body weight per day. A 70kg adult lands around 112mcg. The dose tracks lean body mass rather than total weight, so it isn’t a straightforward multiplication in people who are significantly overweight.
Prescribers often start at full replacement in younger patients with no heart disease, because it gets to target faster with no particular downside.
In older adults, and in anyone with coronary artery disease, the approach is deliberately cautious — typically 25mcg or 50mcg daily, increased in 25mcg steps every four to six weeks. Thyroid hormone raises myocardial oxygen demand. Correcting a long-standing deficit too quickly can precipitate angina, arrhythmia or infarction in a heart with limited reserve. “Start low, go slow” is a real safety principle here, not excessive caution.
Subclinical hypothyroidism — raised TSH with normal free T4 — is genuinely contested. Most guidelines suggest treating when TSH exceeds 10 mIU/L. Between the upper limit of normal and 10, decisions are individualised around symptoms, thyroid antibody status, age, cardiovascular risk and pregnancy plans. Several large trials in older adults, including the TRUST trial, found no meaningful symptomatic benefit from treating mild subclinical hypothyroidism, which has made many clinicians more conservative than they were a decade ago.
After thyroidectomy or radioiodine, the gland produces nothing, so full weight-based replacement is needed from the start.
The six-strength ladder exists because thyroid dosing is genuinely fine-grained. The 88mcg tablet is not an oddity — the gap between 75 and 100 is large enough that a substantial number of people sit uncomfortably in it. Alternate-day dosing between two strengths is also a legitimate technique for achieving in-between averages.
Absorption: the part that actually determines whether it works
Levothyroxine is absorbed in the jejunum and upper ileum, and absorption is inefficient and easily disrupted. Only about 70 to 80% of an oral dose gets across even in ideal conditions. This is where most treatment failures come from — not the dose, the absorption.
The core rule
Take it on an empty stomach with water, 30 to 60 minutes before food or any other drink, at the same time every day.
An evening alternative — at bedtime, at least three hours after the last meal — has been tested in randomised trials and works at least as well, sometimes slightly better. It suits people who can’t manage a morning fast. What matters far more than which slot you choose is that you keep the same one every day.
Coffee
This deserves its own heading because it’s the most common unrecognised cause of a stubbornly raised TSH. Coffee taken with or shortly after levothyroxine reduces absorption by roughly 30%. Espresso appears worse than filter. The effect is large enough to look like treatment failure and prompt an unnecessary dose increase.
If you take your tablet in the morning, coffee waits an hour. This is not a minor optimisation.
Drugs and supplements that must be separated by four hours
These bind levothyroxine in the gut and carry it straight through:
- Calcium supplements, and calcium-containing antacids
- Iron supplements
- Magnesium supplements
- Aluminium and magnesium antacids
- Sucralfate
- Cholestyramine and colesevelam
- Sevelamer and other phosphate binders
- Multivitamins containing any of the above
Four hours, in either direction. This is the most frequently missed instruction in thyroid care, and multivitamins are the usual culprit because people don’t think of them as medication.
Drugs that reduce absorption or increase clearance
- Proton pump inhibitors (omeprazole, lansoprazole and similar) — reduce gastric acid, which levothyroxine needs for dissolution. Starting or stopping a PPI often requires a dose review.
- Oestrogen — combined oral contraceptives and HRT raise thyroid binding globulin, increasing the bound fraction and reducing free hormone. Starting either commonly means needing more levothyroxine.
- Carbamazepine, phenytoin, phenobarbital, rifampicin — induce hepatic enzymes and accelerate breakdown.
- Sertraline — reported to increase requirements in some patients.
Foods
Soy protein, high-fibre foods, walnuts and grapefruit juice all reduce absorption. You don’t need to avoid them — just don’t take them within an hour of your tablet.
Coeliac disease, H. pylori, and atrophic gastritis
All three impair absorption and are meaningfully overrepresented in people with autoimmune thyroid disease. If someone needs an unusually high dose with no obvious explanation, coeliac screening is a standard next step.
Monitoring
TSH is the primary test. Free T4 is measured alongside it in various situations, and free T3 occasionally, but TSH does the heavy lifting.
Recheck six to eight weeks after any dose change. This is not arbitrary. With a seven-day half-life, reaching a new steady state takes roughly five half-lives — about five weeks — and the pituitary then needs further time to recalibrate. Testing earlier gives a number that hasn’t finished moving, and acting on it leads to overcorrection and a frustrating oscillation.
Once stable, annually is standard, with extra tests whenever something changes.
Timing on test day: don’t take your tablet before the blood draw. It causes a free T4 peak a few hours after dosing that can be misread as over-replacement. Take it after the test.
Target ranges. Most laboratories quote roughly 0.4 to 4.0 mIU/L for TSH. Most treated patients are aimed at the lower half, somewhere around 0.5 to 2.5, though this is individualised. Older adults are often deliberately kept higher. Pregnancy and thyroid cancer follow entirely separate targets.
Why your dose changes
It’s normal for a stable dose to need revision over the years.
Weight change. Dosing is weight-based. Substantial gain or loss shifts the requirement, and this includes weight lost after bariatric surgery — which also alters absorption independently.
Pregnancy. The most important item on this list. Requirements typically rise 30 to 50%, and they rise early — often by weeks four to six, before many people have booked a first appointment. Foetal brain development depends on maternal thyroid hormone through the first trimester, because the foetal thyroid doesn’t function until around week 12. Untreated or under-treated maternal hypothyroidism in early pregnancy is associated with impaired neurodevelopment.
Anyone on levothyroxine who is pregnant or planning pregnancy should have this conversation before conceiving, and have thyroid function checked as soon as pregnancy is confirmed. A common pre-agreed plan is to increase the dose immediately on a positive test — often by taking two extra doses per week — and then test. Requirements return to pre-pregnancy levels after delivery.
Ageing. Requirements tend to fall gradually from around age 60.
Progression of the underlying disease. In Hashimoto’s, residual thyroid tissue continues to be destroyed over years. Someone who started on 50mcg may genuinely need 125mcg a decade later. That’s disease progression, not treatment failure.
New medication. Any of the interacting drugs above.
New gastrointestinal disease. Coeliac, atrophic gastritis, inflammatory bowel disease.
Brand or formulation change. Different manufacturers’ products are bioequivalent by regulatory standard, but that standard permits a tolerance band which, on a narrow-therapeutic-index drug, occasionally produces a noticeable shift. Most people switch without incident. If you’re sensitive to it, staying with one consistent product is reasonable, and a TSH check six to eight weeks after any switch is sensible.
Getting it wrong in either direction
Over-replacement
Suppressed TSH with symptoms of thyrotoxicosis: palpitations, tremor, heat intolerance, sweating, anxiety, insomnia, weight loss, diarrhoea.
The two consequences that matter long-term:
Atrial fibrillation. Sustained TSH suppression significantly raises AF risk, particularly over 60. AF in turn raises stroke risk substantially.
Bone loss. Excess thyroid hormone accelerates bone turnover. Long-term over-replacement is associated with reduced bone density and increased fracture risk, especially in postmenopausal women.
Both develop silently over years. This is the central reason self-adjustment is a bad idea: taking a bit more because you feel tired is a strategy with no feedback signal until something breaks.
Under-replacement
Persistently raised TSH: the original symptoms continue, plus raised LDL cholesterol and, if severe and prolonged, cardiovascular risk and rarely myxoedema.
By far the commonest causes are the absorption issues above and inconsistent adherence — not an inadequate prescription.
When you feel unwell but the numbers are normal
This is a real and genuinely contested area, and it deserves an honest account rather than dismissal from either direction.
A meaningful minority of people on levothyroxine with a TSH squarely in range continue to report fatigue, cognitive slowing, weight difficulty and low mood. Estimates vary widely, but this is not a rare complaint and it is not imaginary.
The mainstream clinical position is that TSH is the best available marker of tissue thyroid status, that most residual symptoms have other causes worth investigating, and that trials of combination T4/T3 therapy have largely failed to show consistent benefit over T4 alone. Major endocrine society guidelines therefore continue to recommend levothyroxine monotherapy as standard, while several now acknowledge that a supervised trial of combination therapy may be reasonable in selected patients who remain symptomatic.
The counter-position, argued by patient organisations and a number of clinicians, is that TSH is a pituitary measure rather than a tissue measure; that a subset of people convert T4 to T3 poorly — with deiodinase polymorphisms such as DIO2 proposed as one mechanism; that trials have been short and underpowered to detect benefit in that subset; and that some patients report clear improvement on combination T4/T3 or on desiccated thyroid extract.
The honest summary is that the evidence does not currently support combination therapy as routine practice, and does not conclusively rule out benefit in a subgroup that we cannot yet identify prospectively. Research is ongoing.
What’s worth doing in the meantime, if this is you:
- Confirm absorption is actually optimised — the coffee and calcium issues above account for more “treatment-resistant” cases than anything exotic
- Check ferritin, B12, folate and vitamin D; deficiencies are common in autoimmune thyroid disease and produce overlapping symptoms
- Screen for coeliac disease
- Consider whether TSH is in range but at the upper end, where some people do feel better with modest adjustment
- Look properly at sleep, depression, anaemia, perimenopause and sleep apnoea, all of which mimic hypothyroidism and all of which are treatable
- If symptoms persist after all that, ask for endocrinology referral rather than self-experimenting
Desiccated thyroid extract and self-sourced T3 are where this most often goes wrong. Both deliver T3 in proportions that don’t match human physiology, both produce supraphysiological peaks, and both are frequently taken without monitoring — which brings the AF and bone risks above without anyone watching for them.
Common questions
Missed a dose? Take it when you remember, same day. If you don’t remember until the next day, take that day’s dose as normal. Some prescribers advise doubling the following day. With a seven-day half-life, one missed dose is minor. A pattern of missed doses is not.
Can I take it at night? Yes, if it’s at least three hours after eating and you’re consistent.
Do I need this for life? Usually. Hashimoto’s and post-surgical hypothyroidism are permanent. Some transient thyroiditis after pregnancy or viral illness does resolve.
Will it help me lose weight? Only if you’re genuinely hypothyroid, and even then only modestly — most of the initial loss is retained fluid. Levothyroxine is not a weight-loss drug and using it as one is how people acquire AF and osteoporosis.
Can I take it with my other morning medications? Not if any of them are on the four-hour list. Otherwise, keep the one-hour food gap and it’s generally fine.
Does it interact with biotin? Biotin doesn’t affect the drug, but high-dose biotin supplements interfere with the laboratory assay and produce falsely abnormal results. Stop biotin two days before testing.
Should I take iodine or selenium? Iodine is unnecessary in iodine-replete countries and excess can worsen autoimmune thyroid disease. Selenium has modest evidence for reducing thyroid antibody levels in Hashimoto’s, without clear symptomatic benefit. Neither replaces levothyroxine.
The short version
Take it consistently, on an empty stomach, an hour before coffee. Keep calcium, iron and multivitamins four hours away. Retest six to eight weeks after any change, then yearly. Expect the dose to shift over the decades — that’s the disease moving, not the treatment failing. Tell your prescriber immediately if you’re pregnant or planning to be. And if you still feel unwell with normal results, investigate properly rather than adjusting the dose yourself, because the risks of over-replacement don’t announce themselves until years later.
Browse the thyroid care range, or read our guide to hormones in plain language.
This article is general health information and not individual medical advice. Levothyroxine requires a prescription and ongoing blood test monitoring in the UK, US, Australia and most other countries, and dosing decisions should be made by a prescriber with access to your thyroid function results. Do not start, stop, or change the dose of levothyroxine without medical supervision — particularly if you are pregnant, planning pregnancy, over 60, or have any history of heart disease.
