Rather than having your thyroid checked, have you ever had your Hb level checked? If this is low, then you will feel tired, as you do not have enough circulating oxygen in your blood, due to lack of red blood cells.
If this is the case, then you should be taking a Vitamin B complex and eating lot of fruit, veg and some red meat to boost your Hb. Even though your Hb is normal, this does not necessarily mean that you do not have a problem. Your iron stores can also be low (Ferritin) levels and this can lead to tiredness too. Try eating some red meat and fruit and veg. Try liver, as this is high in Vit B and try taking a B complex vitamin to see if this helps.
If this does not help, then go and have some further blood tests to make sure. Tiredness can also be caused by infections such as Mono or a simple virus, that could take you a little time to get over.