A Thyroid Stimulating hormone (TSH) of 6.5 is likely subclinical hypothyroid (normal range is about 0.4 to 5.5). Did they increase your levothyroxine (synthroid) to see if that decreases your symptoms? TSH is produced by the pituitary gland at the base of the brain and its use is to regulate thyroid hormones. Doctors tend to embrace T3 and T4 over TSH. When my TSH is around 3 my T3, T4, and free T3 and T4 levels are still normal range, even though I have hypothyroid symptoms of fatigue, GI problems, and weight gain. I take D3 because that is also low (partly I'm 69 and have celiac); I get enough sun. https://my.clevelandclinic.org/health/diseases/23544-subclinical-hypothyroidism https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5922674/ https://medlineplus.gov/lab-tests/tsh-thyroid-stimulating-hormone-test/#:~:text=The%20pituitary%20gland%20makes%20thyroid,your%20thyroid%20to%20work%20harder.