Yes, the good news is there are a few laboratory tests that can be used to determine what causes you to be tired or have low Energy .

The first test that I recommend is thyroid function.
The proper function of your thyroid hormone is vital for normal energy production, proper body temperature, ideal body weight and overall vitality. The tests that are usually ordered are TSH (thyroid stimulating hormone), FT4 (free T4), FT3 (free T3) and the body temperature test.
I usually don’t endorse the blood tests especially TSH because a lot of people with underlying thyroid conditions may show so called normal results even though they still feel tired. The reason for this is the reference range is too broad (0.3-5.0) and in fact two years ago there was research showing that it should be lowered.

The Body Temperature Test for Thyroid Function:
I favour the body temperature test. Take an oral thermometer and put it under your armpit before you get out of bed and repeat this for three straight days. Your temperature range should be between 97.8F to 98.2F. Take the average of the three days to see if it falls within the range. If it’s below 97.8 then you have hypothyroidism (low thyroid) and if it’s higher than 98.2, than you have hyperthyroidism.

Measure ferritin and iron levels:
Another test that can be done to determine a possible cause of low energy is serum Iron and ferritin. Iron is needed to help produce adequate amounts of red blood cells to carry oxygen throughout the body and supply energy to the body.
The difference between iron and ferritin is that iron is a measure of the amount of iron present in the liquid portion of blood, while ferritin is a measure of the amount of stored iron. Low amounts of iron in the blood indicate anemia which may explain your lack of energy.

Vitamin B-12:
Vitamin B12 deficiency is also related with low energy, triggered by anemia or low iron. A simple blood test can be done to see the nutritional status of B12.

Complete Blood Count:
Another test is the complete blood count (CBC). This test measures immune system function by looking at red and white blood cells. An under-functioning Immune system may also be a cause for low energy, causing stress and leading to a compromised immune system.

The last test I would like talk about is cortisol. Cortisol is the hormone we produce when our bodies are under Stress . Short term production of this hormone is fine, however, chronic stress leads to over-production which then leads to fatigue because of the constant fighting within our bodies.
Basically, excess cortisol can lead to overproduction of insulin and glucose as related to your eating habits, resulting in high blood sugar and low energy.

Nadeem Aslam, B.Sc. researches and recommends new products for Nature's Source.