"The idea is to measure blood levels of T4 and TSH. In the typical person with an under-active thyroid gland, the blood level of T4 (the main thyroid hormone) will be low, while the TSH level will be high. This means that the thyroid is not making enough hormone and the pituitary recognizes it and is responding appropriately by making more Thyroid Stimulating Hormone (TSH) in an attempt to force more hormone production out of the thyroid. In the more rare case of hypothyroidism due to pituitary failure, the thyroid hormone T4 will be low, but the TSH level will also be low. The thyroid is behaving "appropriately" under these conditions because it can only make hormone in response to TSH signals from the pituitary. Since the pituitary is not making enough TSH, then the thyroid will never make enough T4. The real question in this situation is what is wrong with the pituitary? But in the typical and most common form of hypothyroidism, the main thyroid hormone T4 is low, and the TSH level is high. "
When I was diagnosed my T3 and T4 levels were at the very bottom of the range and my TSH was 6 something.
After I started taking thyroid hormone my TSH went down to 2 something, although my T3 and T4 increased only a little bit. I felt a bit better but still a zombie most days. I'm now with a new doctor who doubled then tripled my dose, so we'll see how it goes from there.
His T4 is at the lower end of the range (though not rock bottom). His TSH is also low. According to conventional medicine, if he is hypothyroid, then it would be due to the rare form pituitary failure. However my current, unconventional doctor doesn't believe that we know enough about the body and the thyroid to really say what causes hypothyroidism.
Get his T3 tested if you can. T3 and T4 work together. You can't fully understand his results without a T3 reading.
As a last note, please get his Vitamin D tested. Vitamin D deficiency is really very common in the UK (indoor lifestyle plus overcast days plus winter). "They" recommend 30 minutes a day in noonday sun, but what "they" don't tell you is that is only enough IF you live in a sunny area near the equator AND if you were getting full body sun exposure (e.g. running around naked!). Your son will need to spend a lot more time outside and/or supplement with high doses of vitamin D3.
With the symptoms you're describing your son is probably vitamin D deficient or at the low end of the range. Most of us feel better when we are the very top of the vitamin D range. Vitamin D helps the thyroid too!