In the past I used often to see someone who claimed that levothyroxine is one of the safest medicines available. Given the staggeringly high dose reported below, it bears that out. (I find it difficult to think of other medicines that you could take well over a thousand times the largest dose you ever usually see and live to tell. Maybe B12? 
We are possibly more used to reading of the possibility of poisoning from just double-dosing of paracetamol for a day.)
We regularly see members ask what would happen if they took an extra tablet by accident. Well, in virtually all cases, the worst that would happen is that you might not sleep as well and could feel a little over-dosed.
Of course, long-term over-dosing is a completely different issue.
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
1NYC Poison Center, NY 10016, USA.
Abstract
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]
ncbi.nlm.nih.gov/pubmed/105...
[ I had completely forgotten, and should have looked, but this story was posted some time ago:
NDT sublingually.
Feeling sad.
