Levothyroxine Side Effects: Signs Your Dose Is Too High or Too Low

Levothyroxine is the same hormone your thyroid makes, so most of its side effects are really signs that the dose is a bit too high or a bit too low. Too much can cause a racing heartbeat, shakiness, anxiety, poor sleep, sweating and weight loss. Too little can cause tiredness, feeling the cold, constipation and weight gain. Some symptoms appear on both lists, which is why a thyroid test, not guesswork, decides whether your dose needs changing.

If you have just started levothyroxine, had your dose changed, or recently had thyroid surgery and feel "off", here is what the Australian leaflet and the research say, what needs urgent attention, and what to do next.

When to get help straight away

The Australian consumer leaflet lists serious side effects that mean you should call your doctor straight away or go to the emergency department at your nearest hospital: chest pain, an irregular heartbeat, shortness of breath or rapid breathing, muscle weakness and cramps, unusual movements including tremor, and allergic reactions such as a skin rash.

For chest pain that is severe, getting worse, or has lasted longer than 10 minutes, healthdirect advises calling triple zero (000) and asking for an ambulance. If you think you have taken too much levothyroxine, the leaflet says to call the Poisons Information Centre on 13 11 26, even if you feel fine.

Why most side effects are really dose signs

Because levothyroxine replaces a hormone your body already uses, the Australian prescribing information explains that too large a dose increase, or too high a dose, can cause the symptoms of an overactive thyroid. healthdirect puts it simply: side effects of thyroid medicines are rare, so long as you are taking the correct dose. The American Thyroid Association notes that the vast majority of people tolerate it without problems.

The consumer leaflet also lists less serious effects to speak to your doctor about if they worry you, including nervousness, poor sleep, headache, diarrhoea, feeling hot, sweating, flushing, weight loss, period changes, tiredness and hair loss. Most of these match what the prescribing information describes as signs of too high a dose.

Signs your dose may be too high

• A racing, pounding or irregular heartbeat
• Shakiness or a tremor
• Feeling anxious, restless or wired
• Trouble sleeping
• Feeling hot, sweating more than usual
• Diarrhoea
• Weight loss you didn't intend

Signs your dose may be too low

• Tiredness and low energy
• Feeling the cold
• Constipation
• Weight gain
• Dry skin and thinning hair
• Low mood or slowed thinking
• Muscle aches or cramps

Why symptoms alone can't tell you which way

Tiredness, poor sleep, hair loss, period changes and muscle weakness can appear whether your dose is too high or too low. In a large US study of more than 25,000 people, individual symptoms were poor at predicting thyroid levels. That is why the answer is a thyroid test, not adjusting your own dose. Never change, skip or split your dose yourself.

Feeling hot, wired or shaky after thyroid surgery

This is common, and it often means your first dose overshot. After a total thyroidectomy, the starting dose is usually worked out from your body weight, and it is an estimate. A 2025 review of 11 studies and more than 2,500 people found that only about a third had their levels in range at the first check after surgery, whichever dosing method was used. In individual studies, between about a quarter and a half started on a dose that was too high, and reaching the right dose can take a few months.

If you feel hot, shaky, sweaty, wired or can't sleep in the first weeks after surgery, tell your surgeon, GP or endocrinologist rather than waiting it out. Our guide to how long a dose change takes explains why levels need about six weeks to settle after each change.

After thyroid cancer, your team may deliberately keep your TSH lower than usual, depending on your cancer's risk, and that can bring mild symptoms of having more thyroid hormone. That target is personal and reviewed over time, so talk it through with your team rather than comparing it with anyone else's.

Long term: why a dose that's too high matters

A TSH pushed very low for years is linked with atrial fibrillation (an irregular heart rhythm) and weaker bones, especially in older people and after menopause. In a large US study of people over 60, a very low TSH was linked with atrial fibrillation in 28% over 10 years, compared with 11% of people with a normal TSH. A Scottish study of more than 17,000 people on thyroxine found more heart problems and fractures when TSH was suppressed, but not when it was only slightly low. Doses drift off target in both directions quite often, which is why regular checks matter. Our guide to what a normal TSH means explains more.

Is it an allergy?

Rarely. The Australian prescribing information says allergy to thyroxine itself has been described only rarely. When people do react, it is usually to an inactive ingredient, such as a dye or filler, rather than the hormone. Brands differ in their inactive ingredients, so if you think you are reacting to your tablets, talk to your doctor and pharmacist rather than switching or stopping on your own. Our guide to switching levothyroxine brands in Australia covers the differences.

Things that look like side effects but aren't

Sometimes a change in how you feel comes from something affecting how much levothyroxine you absorb, or how your test reads:

• Calcium and iron supplements taken with your tablet can lower absorption. In one study, calcium taken alongside it pushed TSH from 1.6 to 2.7. Keep mineral supplements and powders several hours away from your tablet.
• Some reflux medicines can lower absorption too. Tell your doctor if you start or stop one.
• High-dose biotin, common in hair supplements, can make your test results read wrong. See our guide to biotin and thyroid tests.
• A new brand can shift your levels slightly, so a test about six weeks after a switch is common advice.

Does levothyroxine cause weight gain?

Weight gain is not listed as a side effect on the Australian leaflet. It appears as a symptom of an underactive thyroid itself, while weight loss is the listed side effect of too much. The leaflet also says levothyroxine should not be used for weight loss. When people start treatment for an underactive thyroid, recent reviews found the average weight change is small, around 1.4 to 2.2 kg. If your weight is changing a lot, ask your doctor to check your levels and look for other causes. Our guide to weight and an underactive thyroid goes further.

What about taking it at night?

Some people take their tablet at bedtime instead of in the morning. A trial found it works at least as well, and can lower TSH slightly, but quality of life and symptoms didn't change. Because it can shift your levels, it is a switch to make with your doctor, and it isn't a fix for side effects.

Normal results, but you still feel unwell?

That is a different problem from side effects, and it is recognised. The American Thyroid Association says these symptoms should be acknowledged and other causes looked for. healthdirect suggests talking to your doctor, who may refer you to an endocrinologist. Our guide to why levothyroxine doesn't fix everything covers what to check.

What to do

1. If you have chest pain, an irregular heartbeat or trouble breathing, get help straight away.
2. For other symptoms, write down what you notice and when it started.
3. Ask your doctor for a thyroid test, ideally about six weeks after any dose or brand change.
4. Mention every supplement and medicine you take, including biotin, calcium, iron and reflux medicines.
5. Never change, skip or split your dose yourself.

Most levothyroxine side effects are the dose talking. Let a test, not a guess, decide what changes.

Frequently asked questions

What are the side effects of levothyroxine?

Most are signs the dose is too high: a racing heartbeat, tremor, anxiety, poor sleep, sweating, feeling hot, diarrhoea and weight loss. Serious effects such as chest pain, an irregular heartbeat or an allergic rash mean calling your doctor straight away or going to the emergency department.

How do I know if my levothyroxine dose is too high?

Common signs are a racing or irregular heartbeat, shakiness, feeling anxious or wired, poor sleep, sweating, feeling hot and unintended weight loss. Some symptoms overlap with a dose that is too low, so ask your doctor for a thyroid test rather than adjusting your dose yourself.

Can levothyroxine cause weight gain?

Weight gain is not listed as a side effect on the Australian leaflet. It is a symptom of an underactive thyroid, which can mean the dose is too low. Weight loss is the listed side effect of too much. If your weight changes a lot, ask your doctor to check your levels.

Why do I feel hot and anxious after my thyroidectomy?

It is common for the first levothyroxine dose after surgery to be too high: a 2025 review found only about a third of people had levels in range at their first check. Tell your surgeon, GP or endocrinologist. Don't wait it out or change the dose yourself.

Can you be allergic to levothyroxine?

True allergy to thyroxine itself is rare. When people react, it is usually to an inactive ingredient such as a dye or filler, and brands differ in these. If you think you are reacting to your tablets, talk to your doctor and pharmacist before switching or stopping.

The honest takeaway

Levothyroxine is the same hormone your body has always used, so most side effects are signs the dose needs fine-tuning, especially early on or after surgery. Get urgent help for chest pain, an irregular heartbeat or trouble breathing. For everything else, note what you feel, ask for a test, and let your doctor adjust the dose with you.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670-751.
Ringel MD, Sosa JA, Baloch Z, et al. 2025 American Thyroid Association management guidelines for adult patients with differentiated thyroid cancer. Thyroid. 2025;35(8):841-985.
Chiardi I, Croce L, Caccavale P, et al. Establishing the adequate levothyroxine dose after total thyroidectomy: a systematic review with meta-analysis. Journal of Clinical Endocrinology and Metabolism. 2025;110(11):3301-8.
Jin J, Allemang MT, McHenry CR. Levothyroxine replacement dosage determination after thyroidectomy. American Journal of Surgery. 2013;205(3):360-3.
Elfenbein DM, Schaefer S, Shumway C, et al. Prospective intervention of a novel levothyroxine dosing protocol based on body mass index after thyroidectomy. Journal of the American College of Surgeons. 2016;222(1):83-8.
Ojomo KA, Schneider DF, Reiher AE, et al. Using body mass index to predict optimal thyroid dosing after thyroidectomy. Journal of the American College of Surgeons. 2013;216(3):454-60.
Flynn RW, Bonellie SR, Jung RT, et al. Serum thyroid-stimulating hormone concentration and morbidity from cardiovascular disease and fractures in patients on long-term thyroxine therapy. Journal of Clinical Endocrinology and Metabolism. 2010;95(1):186-93.
Lillevang-Johansen M, Abrahamsen B, Jørgensen HL, et al. Over- and under-treatment of hypothyroidism is associated with excess mortality: a register-based cohort study. Thyroid. 2018;28(5):566-74.
Sawin CT, Geller A, Wolf PA, et al. Low serum thyrotropin concentrations as a risk factor for atrial fibrillation in older persons. New England Journal of Medicine. 1994;331(19):1249-52.
Canaris GJ, Manowitz NR, Mayor G, Ridgway EC. The Colorado thyroid disease prevalence study. Archives of Internal Medicine. 2000;160(4):526-34.
Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283(21):2822-5.
Bolk N, Visser TJ, Nijman J, et al. Effects of evening vs morning levothyroxine intake: a randomized double-blind crossover trial. Archives of Internal Medicine. 2010;170(22):1996-2003.
Wolde Sellasie S, Ossola N, Piticchio T, et al. Levothyroxine therapy for euthyroidism restoration results in clinically insignificant weight and anthropometric changes: a systematic review and meta-analysis. International Journal of Obesity. 2026.
Saravanan P, Chau WF, Roberts N, et al. Psychological well-being in patients on 'adequate' doses of l-thyroxine: results of a large, controlled community-based questionnaire study. Clinical Endocrinology. 2002;57(5):577-85.
Eutroxsig, Oroxine and Eltroxin (levothyroxine sodium) Product Information and Consumer Medicine Information. Australian Commission on Safety and Quality in Health Care, Medicine Finder. 2024-2025.
healthdirect Australia. Thyroid medicines; Chest pain. healthdirect.gov.au. 2025.