Doctors usually assume that all hypothyroidism is primary, due to Hashimoto's Thyroiditis, which is characterized by increasing levels of TSH, in an attempt to stimulate output of thyroid hormone from the thyroid gland. Hypothyroidism can also be due to dysfunction in the hypothalamus/pituitary system resulting in insufficient output of TSH to adequately stimulate the thyroid gland. Central is characterized by relatively low levels of TSH along with low levels of Free T4 and free T3, which are the biologically active thyroid hormones. Doctors erroneously think that central hypothyroidism is rare, but it is just rarely diagnosed due to the total reliance on TSH testing for diagnosis. That is why you always need to be tested for Free T4 and Free T3 each time.
What symptoms do you have other than weight gain, depression and anxiety?
TSH is totally inadequate as the primary diagnostic for thyroid status. Diagnosis for potential hypothyroidism should be an integrated approach, starting with a full medical history and then an evaluation for symptoms that occur more frequently with hypothyroidism than otherwise, supported by expanded testing beyond just TSH. You should always make sure they test for Free T4 and Free T3 every time you go for tests. Initially you should also be tested for Reverse T3 and cortisol to assure they are not contributing to your condition. Also, hypothyroid patients are frequently deficient in Vitamin D, B12 and ferritin those should also be tested and then supplemented as needed to optimize. D should be at least 50 ng/mL, B12 in the upper part of its range and ferritin should be at least 100.
So, before further discussion please tell us about the symptoms you have.