I shall give you some links and your query is one of the commonest i.e. unwexplained weight gain when hypothyroid.
Have you had a recent blood test for your thyroid hormones, if not make one for the earliest time possible, fasting (you can drink water) and allow a gap of 24 hours between last dose and test and take afterwards. Ask for B12, Vit D, iron, ferritin and folate if you've not had them done for some time.
Ask GP for a Full Thyroid Function Test - probably wont - but ask anyway.
TSH, T4, T3, Free T4 and Free T3. The FT3. You can get the FT4 and FT3 privately if lab wont do them. This is an excerpt from the following link:-
FT3 = FREE T3
T4 converts to T3 and is the only thyroid hormone actually used by the body's cells.
The approx. reference range for Free T3 is 4 to 8.3
We at Thyroid UK believe that you need to know your Free T3 level too because this will often show low if you are not converting, and high if you have blocked receptor cells. Even if you are converting, the body needs the extra T3 that a normal thyroid produces. There has been some research to show that people feel better on a mixture of Thyroxine (T4) and Triiodothyronine (T3). Effects of Thyroxine as Compared with Thyroxine plus Triiodothyronine in patients with hypothyroidism –
T3 is the only active hormone required in all of our receptor cells. T4 is inactive and has to convert to T3.