(Vet-a-mix) Thyro-L is used for correction of conditions associated with hypothyroidism in horses. Thyro-L contains levothyroxine sodium. Each level teaspoon of powder contains approximately 12 mg of levothyroxine sodium, USP. The recommended dosage of Thyro-L is 0.5 to 3.0 mg levothyroxine sodium per 100 lbs body weight given orally once per day, or in divided doses. Levothyroxine sodium.