Two other common components of fatigue to work up:

In patients with a presentation of fatigue, who are thinking of lyme or other infection as contributing, often we have to consider viral reactivation previously discussed here as well as looking at some specific aspects of thyroid function. 

Usually there has already been comprehensive work up from the family doctor for things like anemia, hypothyroidism, pulmonary disease, iron deficiency etc. The thyroid is the harder part: the usual test is a TSH level which stands for thyroid stimulation hormone. This comes from the brain to act upon the thyroid to release T4 - which is levothyroxine, the most commonly prescribed thyroid replacement. 

In many lyme patients and some myalgic encephalitis patients, it is important to consider the conversion of T4 into T3, which is the much more active hormone. In some, this level is low. In others, it is normal, but we also need to consider reverse T3, or the hormone that blocks free T3. In effect, we need to account for the ratio as some patients have much more reverse T3 than usual for their amount of T3.

Dessicated thyroid has T3 and T4, and is often useful. In cases where the reverse T3 ratio is quite abnormal, any extra T4 may not be useful and I will use T3 alone in these patients to help with their brain fog and fatigue.