"Acute systemic thyrotoxicosis can be induced by parenteral
administration (intraperitoneal or intravenous) of 1.0 ug
T3/g BW, a TR-saturating dose of T3. Genomic effects can
be seen as early as 60 minutes and physiologic effects
starting at about 6 hours" according to ATA guidelines. If you're looking for a more chronic study (greater than 5 days), you'll want to stay within 10- to 25-fold the daily production rate. For chronic studies, administration via chow or water (you'll want to measure intake, of course) or subcutaneous pumps/pellets might be more ideal than injections, as they reduce the both the work required and the stress placed on the animal.