You might also be interested to know that there is information suggesting that hypothyroid patients average about 390 calories per day below normal for their age, height, weight, and gender.
Based on the usual formula for estimating a person's Resting Metabolic Rate (RMR), you can estimate the effect of a drop in RMR by dividing the amount by 4.3. So if there is no change in diet or exercise, over an extended period, a drop of even 200 calories per day would account for about 45 pounds (200 calories low in RMR /4.3).
I suspect that you may have a few more symptoms typical of hypothyroidism, but not recognized. Have a look at this link.
As for weight gain, FT3 is the biologically active thyroid hormone that creates metabolic activity at the cellular level. Free T4 is a prohormone, available for conversion to T3. So to feel normal and have normal metabolism for your age, weight and height and gender, you need for your FT4 and FT3 to be adequate to relieve hypothyroid symptoms.
Dosing a hypothyroid patient based on TSH strictly does not work. If you want to know why, I suggest reading my paper in the following link. Thyroid med dosage has to be adjusted as needed to eliminate hypo symptoms, without going so far as to create hyper symptoms. FT4 and FT3 levels are useful to monitor the effects of dose changes. Typically FT4 should be about mid-range, and FT3 at least mid-range, and adjusted from there as needed to relieve symptoms.
It is also important o note that the ATA recommends that the morning dose of thyroid med should be delayed until after the blood draw, to avoid false high results. If you took your med before the blood draw, your test results are higher than actual.
Most hypothyroid patients taking T4 med find that their body does not adequately convert the T4 to T3. This is evident in your results. FT4 is borderline at 40% of its range, while FT3 is only at 26% of its range, which is too low. and if you took your med before the blood draw, your actual levels are even lower. So you are going to need at least some T3 added to your med. If your doctor does not understand this, give a copy of my paper and ask to be treated clinically (as needed to relieve symptoms).
Were you on 100 mcg of Levo at the time of those tests? What are the reference ranges shown on the lab report for the Free T3 and Free T4? What other symptoms do you have?