If you feel o.k. on 100mcg that should be o.k. but if you experience palpitations or too hot I would drop down by 25mcg. Normally, an increase of 25mcg about every six weeks is given until symptoms go and patient feels well.
It's a good idea to have a record of your pulse/temp when first introducing levo. As hypothyroidism patients usually have a low temp and pulse. As levo is increased they rise, more or less, back to normal.
Your next blood test should be at the earliest possible time and don't eat before it but you can drink water. Also leave about 24 hours between your last dose and the test and take it afterwards. This helps keep your TSH at a higher level as doctors are apt to adjust levo to keep us within the 'range'. We need a TSH of around 1 or lower.
Levo should be taken first thing with a glass of water and wait about an hour before eating. Food can interfere with the uptake of thyroid hormones.
If GP hasn't tested Vit B12, Vit D, iron, ferritin and folate and thyroid antibodies ask for these too. We can be deficient.
Always get a print-out with the ranges for your own records and you can post if you have a query.