Hello Angela, many conditions including medical and nutritional deficiencies as well as toxicity can cause these symptoms. However, thyroid disease is extremely underdosgnosed, even when severe, as it often requires extensive blood tests to detect. So you probably had a typical TSH prescreening test. Unfortunately, the ranges on this test are too wide to be much use and it tells only what your pituitary gland is telling your thyroid to do, nothing about actual thyroid production. If you have symptoms and your TSH is near 2, you should probably push for more thyroid tests.
If you have a history of other medical conditions, such as gallbladder surgery, for example, this can also result in weight gain.
Since you have a history of being underweight and are gaining, it's unlikely diabetes, since diabetes most often results in weightloss. Adrenal exhaustion is a candidate if you're under high stress. Many thyroid patients also have adrenal exhaustion.
Vitamin and mineral deficiencies to consider are Bs, D, magnesium, copper, boron, iron to name a few. There are many nutritional deficiencies, so I recommend starting with good quality mineral and vitamin supplements (that include coblimated Bs) to start with.
If your diet is poor, you will have health problems, so it's imoortant to have a good diet. If you do have thyroid disease, diet is extremely important. You need to get enough protein and vegetables. If you have a history of low or poor quality protein, (you really can't get good bio available protein from entails, so vegetarians often suffer from low protein) you can supplement with essential amino acids. These will help your general health. I've found them to be extremely effective.
Exposure to toxic substances particularly bromine and mercury, can cause thyroid disease by interfering in the thyroid feedback loop. So detoxing is extremely important.
Lastly, thyroid disease is rampant and has been estimated to effect as much as 30-50% of women over 30. Depending on the source, there are many different numbers, but you get the picture. Thyroid disease can go undiagnosed for decades.
Thyroid disease can be curbed substantially by diet and lifestyle changes. Treatment of thyroid disease is often not what most people consider effective. At best, medical treatment manages thyroid disease, but is often extremely difficult. From diagnosis to treatment and meds, thyroid patients generally find it extremely difficult to get effective treatment. For this reason, many thyroid patients wind up troubleshooting their entire lifestyle to eliminate other conditions and causes. Hence, my recommendations for diet and supplements, as this will help eliminate deficiencies as the source of your symptoms, and maybe relieve the symptoms.
Hope that helps give you some ideas of how to proceed.