Reconstitution:
Reconstitute the lyophilized levothyroxine sodium by aseptically adding 5 mL of 0.9% Sodium Chloride Injection, USP only. Shake vial to ensure complete mixing. Use immediately after reconstitution. Do not add to other IV fluids. Discard any unused portion.
Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.
Storage / Stability: Store dry product at controlled room temperature 15° to 30°C (59° to 86°F). Protect from light.
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I found this in Micromedex....See the following
1) INTRAVENOUS - PIGGYBACK
a) ........... Stability tests have indicated that levothyroxine is stable for up to 48 hours following reconstitution, if retained in the original vial.......