Is Your Levothyroxine Dose Too High or Too Low? The Signs to Know

Levothyroxine replaces a hormone your body needs in a fairly precise amount, so a dose that is a little too high or a little too low can each make you feel unwell, in almost opposite ways. Too much speeds things up: racing heart, shakiness, poor sleep, loose stools. Too little slows them down: tiredness, cold, constipation, weight gain. The two lists overlap, which is why a blood test, not a feeling, decides whether the dose needs a change.

What are the signs your levothyroxine dose is too high?

The FDA label says the adverse reactions of levothyroxine are "primarily those of hyperthyroidism due to therapeutic overdosage" [C1]. Most of what people call side effects is simply too much thyroid hormone. The label lists [C1]:

• Heart: palpitations, a fast pulse, arrhythmias, higher blood pressure; in serious cases angina or heart failure
• Nervous system: nervousness, anxiety, irritability, mood swings, insomnia, headache, hyperactivity
• Muscles: tremor, muscle weakness, spasms
• Heat and weight: heat intolerance, heavy sweating, fever, bigger appetite, weight loss
• Gut: diarrhea, abdominal cramps, vomiting
• Skin, periods, bone: hair loss, flushing, rash, menstrual irregularities, reduced fertility, lower bone density

The same list includes fatigue [C1]. So feeling tired does not automatically mean your dose is too low.

The reassuring part: a 2026 systematic review of 12 studies (1,817 people) found these effects appeared mainly when TSH was suppressed. In blinded, placebo-controlled trials of people whose TSH was in range, levothyroxine caused no significant difference from placebo [C3]. Our guide to levothyroxine side effects covers that distinction in depth.

What are the signs your levothyroxine dose is too low?

A dose that is too low leaves your body short of thyroid hormone, so hypothyroid symptoms linger or return. MedlinePlus lists the early ones as fatigue or feeling slowed down, weakness, feeling cold, hard stools or constipation, weight gain, pale or dry skin, thin or brittle hair and nails, sadness or depression, joint or muscle pain, and heavier, irregular periods [C2]. If the shortfall is larger or lasts longer, later signs include a slow heart rate, puffiness of the face, hands and feet, hoarseness, slow speech and a low body temperature [C2]. They come on slowly, so many people only notice them in hindsight.

Too high vs too low: the signs side by side

The "too high" column comes from the FDA label [C1], the "too low" column from MedlinePlus [C2].

Sign	Too high (overreplacement)	Too low (underreplacement)	What to ask your doctor
Heart	Palpitations, fast or irregular pulse, higher blood pressure	Slow heart rate (later sign)	"Could my TSH be suppressed? Should my heart rhythm be checked?"
Sleep and energy	Insomnia, hyperactivity, but also fatigue	Fatigue, feeling slowed down, weakness	"Is this tiredness thyroid-related, or should we look for another cause?"
Temperature	Heat intolerance, heavy sweating, fever	Feeling cold, low body temperature	"Has my TSH moved since my last test?"
Weight and appetite	Weight loss, bigger appetite	Weight gain	"Has my weight changed enough to revisit my dose?"
Bowels	Diarrhea, abdominal cramps	Hard stools, constipation	"Could a new medicine or supplement be affecting absorption?"
Mood	Nervousness, anxiety, irritability, mood swings	Sadness, depression	"Should we check TSH before looking at other causes?"
Skin and hair	Hair loss, flushing, rash	Pale or dry skin, thin or brittle hair and nails	"Is this hair loss from my dose, or something else?"
Periods	Menstrual irregularities	Heavier, irregular periods	"Could my dose explain this, and does it matter if I want to get pregnant?"

Fatigue, weakness and hair changes appear in both columns.

Why symptoms can't decide it, and when TSH is checked

Symptoms are a poor guide on their own. Several appear on both lists [C1][C2], many have causes unrelated to the thyroid, and they lag behind blood levels: the label notes the peak effect of a dose may take 4–6 weeks [C1]. Age blurs the picture too. In a Danish population study of newly diagnosed hypothyroidism, all 13 symptoms studied were more common than in matched controls among people under 50, but only 3 were among people 60 and older [C7].

So TSH (thyroid-stimulating hormone) is the arbiter. When the levothyroxine dose is too high, TSH falls; when it is too low, TSH rises. See understanding your TSH test for how to read the number.

For adults, the label recommends checking TSH 6–8 weeks after any change in dose, then every 6–12 months once the dose is stable, or sooner if your condition changes [C1]. If you take biotin (common in hair and nail supplements), the label advises stopping it at least 2 days before the blood draw because it can distort thyroid test results [C1]. More on timing: how often TSH should be checked.

Why a dose that runs high matters more with age

Over months or years, a dose that runs high is associated with heart rhythm problems and bone loss, both weighing more in older adults.

• Atrial fibrillation. The label states that atrial fibrillation is the most common arrhythmia seen with levothyroxine overdosage in older people and recommends lower starting doses for them and for people with heart disease [C1]. In a Scottish cohort of 17,684 people on long-term levothyroxine, a suppressed TSH (≤ 0.03 mU/L) was associated with more heart rhythm problems (hazard ratio 1.6) and cardiovascular disease (1.37) than a TSH in range [C4].
• Bone loss and fractures. Overreplacement can increase bone resorption and lower bone density, particularly after menopause [C1]. In the Scottish cohort, a suppressed TSH was associated with about twice the fracture risk (hazard ratio 2.02) [C4]. Among 213,511 adults aged 70 or older on levothyroxine in Ontario, doses above about 93 µg/day were associated with 3.45 times the odds of fracture compared with doses below about 44 µg/day [C5].

A dose that runs too low carried risk as well: a TSH above 4.0 mU/L was linked to more cardiovascular disease, rhythm problems and fractures in the same cohort, while a TSH that was low but not suppressed (0.04–0.4 mU/L) was not linked to higher risk [C4]. These are observational studies, so they show associations, not proof of cause.

A dose that runs high is not rare. In a US aging cohort, 9.6% of people taking thyroid hormone had a low TSH versus 0.8% of people not taking it, and in about one-third of participants with follow-up data it lasted at least two years [C6]. More: thyroid and cardiovascular risk and thyroid and osteoporosis.

Why does a dose that used to be right stop fitting?

A dose that worked for years can drift out of range. The label notes the right dose depends on age, body weight, heart condition, other illnesses including pregnancy, other medicines and food [C1]. Common reasons:

• Weight change. Full replacement is calculated per kilogram (about 1.6 µg/kg/day for many adults), so a large gain or loss can shift what you need [C1].
• Pregnancy. Requirements may rise, so TSH is monitored during pregnancy and the dose usually returns to the pre-pregnancy level right after delivery [C1]. Tell your doctor as soon as you are pregnant or planning to be [C1].
• New medicines or supplements. Iron, calcium and antacids reduce absorption and should be taken at least 4 hours apart from levothyroxine; proton pump inhibitors and sucralfate can reduce it too [C1].
• A brand or generic switch. In a survey of members of three US endocrinology societies, 52 of 54 serious adverse events involved a switch from one levothyroxine product to another, usually made at the pharmacy without the prescriber knowing [C8]. These were physician reports, not a controlled study.
• Timing with food and coffee. The label advises an empty stomach, 30–60 minutes before breakfast [C1]. A systematic review found limited but consistent evidence that coffee, soy, fiber, calcium and iron reduce absorption, and that keeping a proper time gap is an effective way to avoid it [C9]. See taking levothyroxine correctly.
• Aging. Older adults usually start on lower doses because of heart risk [C1].

If one of these applies, tell your doctor or pharmacist rather than changing the dose yourself; only a repeat TSH shows where you have landed. If you use Thyra, daily symptom logs let you look for patterns after 7 or more tracked days, alongside your lab results, to bring to the appointment.

When to get urgent care or call your doctor

Get urgent care (call emergency services) for:

• Chest pain
• A very fast, pounding or irregular heartbeat, especially with shortness of breath, dizziness or fainting
• A very low body temperature, slowed breathing or becoming hard to wake, possible signs of myxedema crisis, a rare emergency caused by very low thyroid hormone [C2]

Call your doctor soon if you notice:

• New palpitations, tremor, insomnia or heat intolerance after a dose increase
• Tiredness, feeling cold or constipation creeping back over weeks
• Unexplained weight change or a change in your periods
• Any of these after a new medicine or supplement, or after your tablets changed appearance

Never stop levothyroxine or change your dose on your own.

Frequently asked questions

Can a levothyroxine dose that is too high cause anxiety?

Yes. The FDA label lists nervousness, anxiety, irritability and insomnia among the signs of too much thyroid hormone [C1]. At a dose that keeps TSH in range, blinded trials found no significant difference from placebo, so a TSH test helps tell whether the dose is the reason [C3].

How long after a dose change will I notice a difference?

The label notes the peak effect may take 4–6 weeks and that it may take several weeks to notice symptoms improve [C1]. That is why TSH is usually rechecked 6–8 weeks after a change, not sooner [C1].

What should I do if I took a double dose by mistake?

Call your pharmacist, your doctor or a poison control center (in the US, 1-800-222-1222) [C1]. The label notes overdose symptoms may not appear until several days later [C1]. Ask before changing anything about your next dose, and get urgent care for chest pain or a racing or irregular heartbeat.

Does hair loss mean my dose is too high or too low?

It can be either. Hair loss is listed among the effects of too much levothyroxine, and the label notes partial hair loss can occur in the first few months of therapy and is usually temporary [C1]. Thin, brittle hair is also a sign of too little thyroid hormone [C2]. Your TSH result tells the two apart.

My TSH is normal but I still feel unwell. What does that mean?

The dose is probably not the cause. In blinded, placebo-controlled trials of people with TSH in range, levothyroxine did not cause significantly more of the studied effects, such as palpitations, headache or anxiety, than placebo [C3]. Ask your doctor about other explanations; in older adults especially, symptoms track thyroid status poorly [C7].

Bottom line

Fast heartbeat, tremor, insomnia, heat and loose stools point toward a dose that is too high; tiredness, cold, constipation and weight gain toward one that is too low. Because the lists overlap, bring your symptoms to your doctor and let a TSH test, usually 6–8 weeks after any change, decide the next step. Get urgent care for chest pain or a very fast or irregular heartbeat, and never adjust the dose on your own.