Levothyroxine side effects: Euthyrox, Thyrax and what to expect

Levothyroxine side effects are, in practice, almost always dose effects. The medicine is a copy of the thyroid hormone T4 your body makes itself, so at a suitable dose most women notice very little. Get too much and you see palpitations, restlessness, sweating and poor sleep. Get too little and your old symptoms simply carry on.

That distinction is missing from nearly every article on this. The leaflet lists every possible complaint, but does not add that most of them are a dose story rather than an allergy or an intolerance.

That matters, because one of those is adjustable and the other is not.

What does levothyroxine do?

Levothyroxine replaces the thyroid hormone T4 that your thyroid is not making enough of. Your body then converts that T4 into the active T3. Because it is chemically the same molecule as your own hormone, it does not behave like a foreign drug but like a top-up of what is missing.

The Dutch Farmacotherapeutisch Kompas describes levothyroxine as the medicine most often prescribed for an underactive thyroid. It works slowly: it takes weeks for the level in your blood to stabilise, and often longer before symptoms follow.

That slowness explains a lot of frustration. You take something, feel nothing for three weeks, and conclude it does not work. In reality your blood level has only just set off.

What are the side effects of levothyroxine?

The best known levothyroxine side effects look like the symptoms of an overactive thyroid: palpitations, tremor, sweating, restlessness, irritability, poor sleep and weight loss. They usually mean the dose is above what your body needs. Rarer are rash and itching, which fit hypersensitivity to an excipient.

Complaints that come up often:

• Palpitations or a racing feeling, especially in the morning
• Restlessness, irritability, a short fuse
• Trouble falling asleep
• Sweating more, feeling warm
• Trembling hands
• Headache in the first weeks
• Temporarily more hair shedding

In women who are just starting, those complaints sometimes come from the speed of the increase rather than the final dose. The Kompas notes that building up in steps is standard practice, precisely to limit this. Always discuss symptoms with your GP or pharmacist before changing anything yourself.

Are Euthyrox and Thyrax the same?

Euthyrox and Thyrax both contain levothyroxine as the active ingredient. They differ in excipients, tablet strengths and manufacturer. For most women they do the same thing, but a small proportion notice a difference in how they feel or in their TSH after switching brand.

That switching happened at scale in the Netherlands when Thyrax was poorly available for years and many people had to change. It has been a recurring question in consulting rooms since.

If you notice a change after a brand switch, that is one of the few situations where having your values looked at again by your GP is genuinely useful. Not because one brand is better, but because absorption can differ slightly between formulations.

Why does levothyroxine need an empty stomach?

Levothyroxine is absorbed in your small intestine, and that absorption is sensitive to whatever else is in your stomach. Food, coffee, calcium and iron can bind part of the dose or slow it down. That is why the leaflet says to take it on an empty stomach, well before breakfast.

Here is the part other articles skip: that interaction is not just a feeling, it is measurable. Benvenga and colleagues showed that coffee taken shortly after the tablet clearly disturbs levothyroxine absorption (Benvenga et al., 2008). Your TSH can come out higher on the same dose than it would without the coffee.

What strikes me here: women who say their dose is never right sometimes do not have a dosing problem, they have a breakfast problem. That is a nicer conclusion, because it is something you and your GP can work with.

How to read those values is in thyroid values explained, and per marker in our pages on TSH and free T4.

Why is my hair falling out since starting levothyroxine?

More shedding in the first months after starting is well known and usually temporary. Your hair cycle resets to the new hormone status, which pushes a larger number of hairs into the shedding phase at once. It feels contradictory, because you took the medicine to fix that very complaint.

It usually settles over a few months as new growth comes through. If it persists, something else is often involved, and ferritin is the most common candidate.

We work that out in thyroid and hair loss and in iron deficiency and hair loss in women.

What if you still feel unwell with a normal TSH?

This happens more often than the leaflet suggests. Your TSH sits inside the range, your dose has not changed, and you are still tired. The question then is not immediately whether your dose should go up, but whether something else is producing the same symptoms.

Low ferritin, a vitamin B12 deficiency and a vitamin D deficiency all produce a picture that resembles a badly adjusted thyroid. In women with heavier periods, ferritin is the most common explanation.

Say your TSH is 1.8 mIU/L, neatly mid-range, and your ferritin is 11 micrograms per litre. That conversation is probably not about your thyroid medication. Our Tiredness panel looks at several of those causes at once, and always tired sets out the possibilities.

Are there long-term side effects?

At a suitable dose, levothyroxine is a medicine people use for decades. Long-term concerns are mainly about structurally too much: a persistently suppressed TSH has been linked in research to effects on the heart and on bone density, particularly in older people.

A pooled analysis of two randomised trials in older people with a mildly raised TSH found no clear cardiovascular benefit from treating (Zijlstra et al., 2021). That says nothing about women with a clearly underactive thyroid, where the picture differs, but it does show that more is not automatically better.

For women after menopause, bone is the point to watch. Our article on bone loss after menopause covers it.

What to discuss with your GP

Side effects are easier to place when you can tie them to a date and a value. Note when symptoms began, whether anything changed in brand, dose or timing, and what your last TSH and free T4 were. Bring those results with you.

Never change your dose yourself and do not stop on your own initiative. That is exactly the kind of adjustment a doctor wants the values in front of them for.

If you want a baseline between two checks, our Thyroid Function looks at TSH and free T4, and the broader Thyroid Complete also includes free T3 and anti-TPO. The background to all of this is in our article on Hashimoto.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Benvenga S, et al. Altered intestinal absorption of L-thyroxine caused by coffee. Thyroid, 2008. PMID 18341376
• Zijlstra LE, et al. Levothyroxine Treatment and Cardiovascular Outcomes in Older People With Subclinical Hypothyroidism: Pooled Individual Results of Two Randomised Controlled Trials. Frontiers in Endocrinology, 2021. PMID 34093444
• Ralli M, et al. Hashimoto's thyroiditis: An update on pathogenic mechanisms, diagnostic protocols, therapeutic strategies, and potential malignant transformation. Autoimmunity Reviews, 2020. PMID 32805423
• Farmacotherapeutisch Kompas, product text for levothyroxine
• NHG guidance on thyroid conditions, Dutch College of General Practitioners