When the thyroid gland is under-active (hypothyroidism), metabolism might slow down so that the body does not burn food fast enough to provide adequate energy. So, you may feel fatigue, a daily lack of energy, an unusual or excessive whole-body tiredness not relieved by sleep or rest.
One of the most common side effects of cancer and its treatment is cancer-related fatigue. It is not predictable by tumor type, treatment, or stage of illness, it comes on suddenly, does not result from activity or exertion, often described as "paralyzing" and it might continue even after treatment is complete.
Radio therapy can cause fatigue that increases over time. Usually lasts from three to four weeks after treatment stops but can continue for up to two to three months.