Well the first thing to do here is to recheck the thyroid function tests including the free T3 levels and free T4 levels. Most doctors only do the TSH to monitor the treatment once drugs have been started but in your case it would be advisable to get a complete panel done. If needed reverse T3 can be looked into as well if needed. Now iyt is possible that your problem basically lies in the conversion of thyroid hormone into its active metabolite which is the T3. That is why the T3 levels are important, synthroid doesnt contain t3 hormones but armour thyroid does contain it. if this is the case then after being on armour thyroid for atleast 2-3 months, you should start to notice a decreasing weight.
Secondly if the thyroid is excluded as the cause here then the extra weight gain should be considered to be due to the adrenal glands. It is very likely that there is a BENIGN cortisol producing tumor in the adrenal glands which can cause weight gain, moon like face, joint pains, increased blood glucose levels, increased abdominal fat. This could be due to cushings syndrome and this would need to be diagnosed with salivary cortisol testing as well as 24 hour urinary cortisol levels. In addition a dexamethasone suppression test would also be needed. Abdominal CT scan to look at the adrenals may be advised if the above tests are abnormal. If this is the case, then surgery can solve the problem permanently.
Another cause of this can be associated with other hormonal issues including problems with the growth hormone which can sometimes lead to weight gain inspite of a normal appetite. This again would need hormone testing to be done. your best best is to get tests for the cushings syndrome and also a complete hormonal panel, this will allow your doctor to exclude most of the common causes for weight gain. At the same time he should get a HbA1C level done as well to rule out diabetes as a cause.
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