Thyroid UK

Is overdosing on T4 dangerous?

Some have been asking questions about the dangers of taking too much T4 eg by accident at one go. I'm told there have been suicide attempts by T4 overdosing and it doesn't work. Children have also swallowed their parent's pills without becoming ill. No-one has died from this, because L-T4 is well tolerated and probably the body's T4-T3 conversion shuts down to stop any problems. This is why T4 therapy has some advantages insofar as the body only makes the active hormone T3 that it needs. However of course longterm overtreatment by T4 is to be avoided because the body responds to it and gives you hyper symptoms. But swallowing the lot at once, not to be recommended but not fatal.

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Hi Diogenes, I agree with you about this. I know that some vulnerable patients are given a week's dose of levothyroxine in one go.

However, when I came out of hospital having had my thyroidectomy (and having been ill for 15 years with hypothyroid symptoms and normal blood test results) they prescribed 200mcg per day. I took this on the first day and had massive overactive symptoms that night. I thought my heart was going to jump out of my chest. Not pleasant!

I've never understood why that happened, when we are constantly told that it takes 7/8 days for it to get into your system. Lyn x


I didn't have immediate reactions to 200mcg T4 but the cumulative over replacement, even though dose was reduced in 25mcg increments over 18 months, made me extremely ill.

Several posters have reported immediate effects of increasing, decreasing or missing a dose. I think the information on how long it takes to get into your system is flawed.

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Hello, y our experience was probably due to being given too much T4 to begin with. Gradual increases are always needed to prevent over-response at the beginning of treatment.


Reassuring to know that kid's haven't OD. I've panicked when one of the flipping tablets breaks as I pop it out of the blister, worrying that one of my dogs will find it and eat it.

I have heard of a GP, mistrusting a patient's compliance, insisted on seeing 7 x days of Levothyroxine ingested in the surgery.


I asked my vet about the possible dangers to my dog of me dropping thyroid pills and vitamin and mineral supplements, and he said I shouldn't worry about it unless I'd dropped vast amounts and hadn't picked them up (which wouldn't happen, of course). It doesn't stop me worrying and getting frantic about it if I drop a pill though, and I can't rest until I've found it/them.


I worry about this with my Armour tablets and my cat - might be more serious I suppose because cats are more likely to be hyper??


I wouldn't worry. So long as you don't spill all your tablets on the floor and the cat gets them. One or two won't hurt. But note that the cat is probably 6 times lighter than you and therefore would be more susceptible to a given dose. But unless you've got malevolent designs against him/her don't worry.


Thanks it I think thx cat is way more than 6 times lighter than me :D I will be careful thanks x


My dog takes 400 mcg daily - they need bigger doses don't think one pill will hurt...


Reassuring. If I want them to have a tablet I have to sneak it into a treat.


:-) :-)


Thanks diogenes,

I came across this fairly extreme intentional overdose a while ago:

Vet Hum Toxicol. 1999 Oct;41(5):323-6.

Severe symptoms following a massive intentional L-thyroxine ingestion.

Hack JB1, Leviss JA, Nelson LS, Hoffman RS.

Author information

NYC Poison Center, NY 10016, USA.


L-Thyroxine (T4) is commonly prescribed medication for hypothyroidism in humans and animals. Overdose has generally resulted in limited symptomatology managed with sedatives and beta-adrenergic receptor antagonists. We describe the largest acute T4 ingestion ever reported, which resulted in a profound thyrotoxicosis, resistant to treatment. A 34-y-old man ingested 900 (0.8 mg) tablets of veterinary T4 (720 mg) and was given 60 g of activated charcoal. He became lethargic on post-ingestion days 2 and 3; had vomiting, diaphoresis and insomnia on day 4; on day 5 he "looked like he had too much coffee", began "using a lot of words" and became agitated, assaultive and stopped speaking intelligibly; and on day 6 returned to the hospital combative and confused. He was diaphoretic, mydriatic, hyperreflexic, tremulous, with clear lungs and active bowel sounds, and received activated charcoal, haloperidol, diazepam, and phenobarbital, and was tracheally intubated. During hospitalization he was rehydrated, treated with propranolol and diazepam, but remained continuously tachycardic. On day 12 he became afebrile and his tachycardia resolved. Free T4 levels ranged from > 13 mcg/dL on day 6 to 1.2 mcg/dL on day 12. By discharge (day 15) he had lost 20 kilograms of body weight, but was clinically euthyroid 2 w later. This case suggests that large intentional T4 ingestions should be managed differently than current T4 overdose protocol.

PMID: 10509439 [PubMed - indexed for MEDLINE]

I think I would find it difficult to swallow 900 tablets!

And this accidental overdose:

To the Editor:

A 33-year-old veterinarian with Hashimoto's thyroiditis and hypothyroidism, stable with a levothyroxine replacement dosage (thyroid-stimulating hormone, 0.9; normal, 0.4-5.0 mIU/L) of levothyroxine 50 μg/d, presented with mild anxiety, jitteriness, and insomnia. Repeat thyroid-stimulating hormone was undetectable. Serum total T4 was 15.4 μg/dL (normal, 6.0-10.0 μg/dL), and free thyroxine index was 14.9 (normal, 6-10). On further questioning, the patient realized she had recently run out of her prescribed levothyroxine tablets and had been taking levothyroxine “dog tabs” 0.5 mg/d, thinking that would be the same as the 50-μg tablets she had been prescribed, inadvertently taking 10 times the prescribed dose. Her levothyroxine therapy was discontinued, and repeat thyroid function studies 2 months later revealed a thyroid-stimulating hormone level of 5.1 mIU/L. The patient was restarted on levothyroxine 50 μg/d, and repeat thyroid-stimulating hormone level 2 months later was 1.0 mIU/L. The patient was instructed to take her prescribed levothyroxine tablets and not use her levothyroxine “dog tabs” in the future.

One of the critical issues is that the effects can be minimal on the first day but gradually build up over days.

The idea that a veterinarian could be so adrift in her understanding of doses is worrying. At least for animals. :-)


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