See Regul Toxicol Pharmacol. 2006 Oct;46(1):29-41. Alpha-lipoic acid (racemic form) (ALA), also called thioctic acid (CAS RN 1077-28-7) was not toxic with an acute LD50 >2000 mg/kg. Administration of 61.9 mg ALA/kg/day for 4 weeks to male/female Wistar rats did not show any adverse effects.

In rats, an LD50 of 2,000 mg/kg of body weight was observed.

see:

Pharmacol Rep. 2011;63(4):849-58.

Lipoic acid - biological activity and therapeutic potential.

Gorąca A, Huk-Kolega H, Piechota A, Kleniewska P, Ciejka E, Skibska B.

Best wishes..

I Think you want ID50 dose 

Thanks

I think it is favorable to measure the LD50 of ALA on your experimental animals because there was a wide range of LD50 across species.