I've been incorporating 50mcg of T3 into my cycles for the last couple of years with good results. It may negatively affect gains a little (it's really hard to tell objectively) but there is a noticable decrease in fat and increase in vascularity.
I settled on that value after reading the space flight catabolism studies I posted a while back. In one study they gave either 50mcg or 60mcg a day of T3 to a group of subjects (dose was set to achieve the degree of catabolism observed in space flight). The table below shows that while only 1kg of muscle was lost, 3kg of fat were lost:
Baseline 6 weeks 9 weeks
Lean body mass (kg) 55.9 ± 2.2 54.4 ± 1.91 54.3 ± 1.81
Fat mass (kg) 15.9 ± 2.7 13.2 ± 2.71 12.6 ± 2.51
24-h EE (Cal/day) 2472 ± 110 2747 ± 1061
Sleeping EE (Cal/day) 1576 ± 73 1862 ± 771
Exercise EE (Cal/min) 3.62 ± 0.24 4.00 ± 0.261
The other study compared test+T3 to T3 alone with the following results:
http://jcem.endojournals.org/cgi/content/full/84/1/207
After 28 days of bed rest, the men in the T3 group lost an average of 3.9 kg of body weight (i.e. from 82.0 ± 7.1 to 78.1 ± 7.1 kg). Body weight in the T3+T-treated subjects declined by only 1.0 kg (78.9 ± 4.9 to 77.9 ± 4.9 kg). This treatment x time interaction was statistically significant (P = 0.002). Lean body mass declined by 1.5 kg in the T3 group, whereas the T3+T-treated subjects experienced nearly a 2-kg increase in lean mass (i.e. 1.7 ± 0.9 kg); again, the treatment x time interaction was statistically significant (P = 0.04).
So 50mcg/day along with 200mg of test enanthate yielded a 2kg increase in LBM.
I'm afraid that 100mcg a day might lead to too much muscle mass loss, and 25mcg a day is not enough to increase fat burning because it will just lower your own T3 without inducing temporary hyperthyroidism.