Do you feel tired all the time? Do you drift through most days in a sluggish state – even if you had a full night’s sleep? Unfortunately, because fatigue has a multitude of different causes, it can be hard to track down the culprit. To help understand why you might be feeling constantly fatigued – even after that full night of sleep – here are some common causes of fatigue.

Sleep Quality

Let’s start with the most obvious source, sleep. It is estimated that 60 million Americans persistently suffer from sleep deprivation. Even if you do get the recommended 7-8 hours of sleep each night, various factors – such as your hormone levels – can affect sleep quality, which means you are not as well rested as you should be.

The two key hormones involved in your sleep cycle are melatonin and cortisol. The levels of these two hormones fluctuate throughout the day and are a determining factor in your body’s alternating cycle of sleeping and waking.

Melatonin regulates the body’s internal biological clock, and it is most active in the body in the evening and at night – that is, unless its production is disrupted. Factors such as too much light, working night shifts, or jet lag can disrupt melatonin production in the body, which in turn disrupts your sleep cycle.

Cortisol signals your body to be awake and responsive, and it is ideally most active in the body during the morning. But certain factors, such as chronic stress, lifestyle, diet, and other health issues, can cause unbalanced cortisol levels, which in turn affects your sleep cycle. 

Irregular levels of either hormone could be the culprit for poor sleep. You should consider testing your melatonin and cortisol levels if you:

  • Have trouble falling asleep
  • Wake up during the night
  • Wake up too early
  • Do not feel rested after sleeping
  • Feel tired but wired
  • Experience mid-day energy dips


Stress is an unavoidable part of life. But when your body experiences chronic stress, it takes a toll on your health and can leave you feeling constantly fatigued. Sleep gets disrupted, your immune system becomes vulnerable, and even your cardiovascular system can become strained. 

As noted above, stress-related cortisol spikes at the wrong time can affect sleep and cause fatigue. On the other hand, long-term stress taxes your adrenal glands and can result in depletion of cortisol, which can cause constant fatigue.

Although cortisol is the body’s primary stress hormone for modulating the body’s daily and long-term responses to stress, cortisol is not the only hormone related to stress.

DHEA, the other major hormone released from the adrenal glands, helps balance cortisol, especially when cortisol levels get too high. An individual who has more DHEA than cortisol seems to experience fewer negative effects from the same stressors than a person who has a lower ratio of DHEA to cortisol.

Testing to measure your cortisol and DHEA levels can give you a better understanding of how your body is managing your stress or if you need to do more to manage stress effectively. You should consider testing if you:

  • Feel tired but wired
  • Cannot wake up without strong coffee
  • Crave sugar, especially late in the day
  • Get sick when you are under stress
  • Feel irritable or anxious
  • Are constantly “stressed out”


Your energy levels are determined by what you eat, because your body converts the fats, proteins, and carbohydrates you eat into daily energy. Therefore, if your diet is less than ideal, it could be a cause of constant fatigue. 

A diet that is high in refined carbohydrates, such as pasta, white breads, sugary desserts, and sweetened sodas and fruit juices, can contribute to fatigue.

These types of carbohydrates cause a rapid spike in blood sugar, which leads to a surge of insulin to move sugar out of the blood and into the cells, followed by a drop in blood sugar – all of which can leave you feeling worn out.

Maintaining a healthy blood sugar level is associated with better metabolic health and improved energy. Testing both your insulin level and your hemoglobin A1c level (which reflects your average blood sugar levels over the previous three months) are two ways to monitor how well your body is metabolizing those carbs. Thorne offers two home tests that include the biomarkers insulin and hemoglobin A1c, Weight Management Test and Heart Health Test.  

Fatigue can also stem from what you are not eating.

If you are not eating a well-balanced diet, which includes plenty of colorful vegetables, moderate amounts of fruit, lean protein, and sources of healthy fats like olive oil and fish, you could be deficient in essential nutrients, which is another common cause of fatigue.

Common deficiencies linked to fatigue include vitamin Dvitamin B12folateiron, and zinc

In addition to what you are eating or not eating, how much you eat also affects your energy levels – whether you are overeating or not eating enough. A calorie surplus causes your digestive tract to work overtime, and that can lead to feelings of fatigue. If your body expends more energy than you are putting in because of calorie deprivation, then you are also going to experience weakness.


Iron deficiency anemia is a common cause for feeling weak and tired. In fact, it is the most common nutritional deficiency in the world. As the name implies, it is caused by a lack of iron, one of the key nutrients needed to produce healthy blood.

Without enough iron, your body cannot produce enough hemoglobin, the important substance found in red blood cells that enables them to carry oxygen throughout the body.

Deficiencies of folate and vitamin B12 can also cause anemia and concurrent fatigue. 

The most certain way to know if your fatigue is a result of anemia is a blood test – a complete blood count (CBC). You should consider getting a CBC test if you are:

  • An individual with a malabsorption syndrome, such as Crohn's disease or Celiac disease
  • A woman who has heavy menstrual periods
  • Pregnant or a woman of childbearing age 
  • A person with low stomach acid secretion
  • An individual with poor kidney function 
  • A frequent blood donor
  • On chemotherapy 
  • Elderly

Thyroid Problems

The thyroid gland affects virtually every organ in the body. The thyroid produces hormones that regulate every bodily process – from how fast your heart beats to how efficiently your intestines digest food.

The two main hormones made by the thyroid gland are T3 (triiodothyronine) and T4 (thyroxine).

The production of these two hormones is regulated by another hormone, TSH (thyroid stimulating hormone), which is produced in the brain.

But various health factors can cause your thyroid to make either too much T3 and T4 (hyperthyroidism) or too little T3 and T4 (hypothyroidism) – and either extreme can cause sleep problems, fatigue, and muscle weakness.

Not every individual who has abnormal levels of T3 and/or T4 will experience obvious symptoms, so testing the thyroid’s key hormones – TSH, T3, and T4 – is a reliable way to determine if an overactive or underactive thyroid gland is the cause of your fatigue. You should consider thyroid testing if you:  

  • Are an individual with health conditions (such as elevated LDL-cholesterol, fatty liver, or obesity) that can’t be explained by diet and lifestyle or other health-related factors
  • Are over 40 years old – especially if you are a woman
  • Experience dry or thinning skin, hair, or nails
  • A woman with unexplained infertility
  • Have a sensitivity to cold or heat
  • Have unexplained weight gain
  • Have difficulty losing weight


Hormonal changes leading up to, during, and after menopause can cause excessive fatigue. Estradiol, progesterone, thyroid, and adrenal hormones are involved with regulating energy at the cellular level, and menopause can cause these hormones to fluctuate at random, which is why fatigue is a common symptom for women going through menopause.

You should consider testing your hormone levels related to menopause if you are:

  • Postmenopausal and want to evaluate your hormone levels
  • Of perimenopausal age (typically between 45 and 55)
  • Experiencing symptoms of menopause

Testosterone levels

A low testosterone level is also linked to long-term fatigue. Testosterone is an important hormone not only for men, but for women as well.

Testosterone has several important functions in the body and impacts many areas of health, including bone and muscle health/mass, libido, fat distribution, heart health, blood sugar metabolism, and hair growth/distribution.

Testosterone is also responsible for generating energy, so naturally, lower levels of testosterone in the body could cause feelings of fatigue. 

You should consider testing your testosterone level if you:

  • Are a woman of childbearing age with fertility issues
  • Notice unfavorable changes in body composition
  • Are experiencing abnormal hair growth or loss
  • Are a male experiencing erectile dysfunction
  • Have loss of libido

Finding your reason for fatigue 

As the above all indicates, looking for a cause of your fatigue can be exhausting. So testing can be a reliable and convenient way to identify health factors that could be contributing to your low energy. With Thorne’s at-home testing kits you don’t have to even leave the comfort of your home.

Have a Thorne test kit sent directly to your doorstep, collect your bio sample, and send it off with prepaid shipping. In about 8-10 days, your results will be available online, along with personalized lifestyle recommendations.