I also have hypothyroid. Your levels change based on the time of day, day of your cycle, stress going on in your life, etc. And recovering from having a baby is definitely stress on your body.
I finally got diagnosed with low thyroid about 3 years ago. NOT by any of my doctors, but my nutritionist/herbalist. All of my tests have always come back in the "normal" medical range. For TSH thats between .5 and 5. When I could barely crawl out of bed, my TSH levels were at 1.75 the first time around. Then about 4 months ago, I started having some of the same feelings, lethargy, etc. and my levels were at 1.5. For me, my TSH levels need to be around a 3 to feel normal and healthy.
But all my docs kept saying I was just fine, even though I wasn't. So I after months of searching for answers, I found my nutritionist, Linda Wagner. I sent her copies of my lab results. My TSH, adrenal gland output and pituitary output was low. Which made sense to her because all three organs work together. So if one is off, so will the other two. It took 7 months of naturally cleansing and strengthening each organ in my body one at a time, and focusing on my thyroid, adrenals, and pituitary for 3 of those months to get my body totally back on track. Also a radical change in diet and an increase in exercise.
Then I surprisingly got pregnant with my daughter 7 months to the day of starting my treatment with Linda. Totally unexpected as I had been dealing with infertility for all of my marriage.
My daughter is now 18 months old and I started to feel the signs of my thyroid not functioning correctly when she was about 14 months old. So now I am back to treating it naturally. Back to watching what I eat and starting to get back on the exercise train. It's been 2 months so far of taking the herbs, supplements that Linda recommended and I feel pretty much back to normal.
If google "THyroid, adrenal, pituitary" a bunch of articles will come up so you can read about how the three work together and what happens when one, two or all three are not functioning properly.
Good Luck!
S.