Low energy: what your blood work can show

Feeling flat is one of the most common reasons Australians see a GP, and one of the hardest things to pin down. Sleep, workload and stress explain a great deal of it.

Some of it is measurable, though. A handful of blood markers relate directly to energy production and transport, and testing can rule them in or out. Here is what those markers are and what they reflect.

Iron

Iron is used to make haemoglobin, the protein that carries oxygen around your body. When iron stores fall, oxygen delivery to muscle and brain becomes less efficient.¹

Iron deficiency is more common in people with heavy menstrual bleeding, in vegetarians and vegans, and in endurance athletes. It also develops gradually — stores can be low well before anything shows on a standard blood count, which is why iron studies measure several markers rather than one.

Vitamin B12

B12 is needed for red blood cell production and nerve function, and low levels can cause anaemia.²

Higher risk sits with vegetarians and vegans, older adults, and anyone with a condition affecting absorption such as coeliac disease. B12 and folate are usually measured together, since they do overlapping work and a deficiency in one can mask the other.

Thyroid function

The thyroid regulates metabolic rate. When it is underactive, a range of processes slow down, and low energy is one of the more common consequences.³

A thyroid panel typically measures TSH alongside thyroid hormones. It is worth knowing that TSH runs counter to intuition — a high reading suggests an underactive gland rather than an overactive one, because it is the pituitary's signal asking for more output.

Vitamin D

Mostly made in the skin from sunlight rather than obtained from food, vitamin D is involved in muscle and bone function, and low levels have been associated with tiredness and muscle weakness.⁴

Risk is higher with limited sun exposure, darker skin, and predominantly indoor living. Sun protection remains important regardless — skin cancer risk is not a trade-off worth making, and testing exists precisely so this can be assessed properly.

Blood glucose

Glucose is the body's main fuel. When regulation is impaired, glucose is less available to cells despite being present in the blood, and persistent tiredness can result.⁵

Fasting glucose gives a snapshot; HbA1c reflects an average across roughly the preceding three months. The two answer slightly different questions and are often measured together.

What testing can and can't settle

• It won't diagnose the cause of fatigue. These markers narrow the field. A diagnosis needs a doctor who can take a history and examine you.
• Normal results don't mean nothing is wrong. Plenty of causes of tiredness — sleep disorders, mood, chronic stress, medication effects — do not show up on a blood panel at all.
• It isn't a substitute for seeing someone. Persistent fatigue that has not improved is worth a GP appointment, whatever your results say.
• Don't self-treat off a result. Supplementation, dose changes and treatment decisions belong with a doctor who can see the whole picture.

The bottom line

Blood testing cannot explain tiredness on its own, but it can check the handful of measurable things that commonly contribute — iron, B12, thyroid function, vitamin D and glucose regulation.

Knowing whether those are in order is genuinely useful, either because it points somewhere specific or because it rules something out and directs attention elsewhere.