I agree with shamon86 that getting your physical health care team to check out your thyroid function is a good idea.

The thyroid gland makes a hormone called thyroxine which comes in two forms (T3, relatively active) and T4 (relatively inactive). Production of thyroxine is controlled by a hormone called Thyroid Stimulating Hormone (TSH) which is made by the pituitary gland and production of TSH is controlled by a hormone called Thyrotropin Releasing Hormone (TRH), made by the hypothalamus.

Low levels of blood T3 are often associated with extreme fatigue.

Measurements of blood levels of T3, T4, TSH and TRH are easy to do. Your physical health care team should be able to interpret those measurements and (depending) suggest further investigations and appropriate treatment.

All the best.