Non-specificity is a hallmark of the signs and symptoms of the most common types of traumatic brain injury.

Regardless of whether the injury arises as a sports concussion (SC), a motor vehicle crash, a fall, or other inciting event. Fatigue falls into the category of non-specific symptoms, and is often the descriptor used by a patient when the term “weak” could suffice. One need not be a physician to formulate a list of a host of potential causes for the type of chronic fatigue that might prompt a patient to complain of the problem including a recent concussion.
 

Unsurprisingly, concussions, thought to be the mildest form of traumatic brain injury, offers its own array of potential explanations for ensuing chronic fatigue and/or weakness.

The list of potential causes would start with the most obvious physical injuries. Both external and internal head injuries, ranging from a bump on the head to subdural and epidural hematoma, contusion of the cortex, and injury to white matter tracts visible and invisible in qualitative imaging sequences. Fatigue may also be the product of the primary injury, and the secondary neurometabolic cascade that produces more cellular injury. Resulting from flooding of glutamate, potassium, calcium, adenosine phosphate, and ionic flux causing electrical dysregulation. See Gizza CC, Hovda DA, “The new neurometabolic cascade of concussion”, Neurosurgery. 2014 Oct; 75 Suppl 4:S24-33. (open access); Kutcher JS, Giza CC, “Sports Concussion Diagnosis and Management”, Continuum Review Article” (Minneap Minn); 2014; 20(6): 1552-1569.(open access). (Continuum is the continuing medical education (CME) arm of the American Academy of Neurology. Chronic fatigue may likewise be the product of chronic brain inflammation. AS well as, immunological alterations that can follow a traumatic brain injury. And chronic fatigue after traumatic brain injury may be caused by some combination of “all of the above” and/or injury to the pituitary. Interestingly, it is possible symptoms that make the presentation look like post-concussion syndrome may actually be hypopituitary alone.
 

The pituitary gland is a tiny organ that sits at the base of the brain.

Located in an area sometimes referred to as the “zone of vulnerability” it serves as the location of the fulcrum of the head and neck that absorbs the brunt of force in most acceleration/deceleration injuries. Injury to the pituitary gland can cause a variety of symptoms including fatigue and/or weakness as well as headache and dizzinessThe most common form of traumatically induced injury to the pituitary is growth hormone deficiency (GHD). Along with fatigue and weakness, hypothyroidism from pituitary dysfunction can also result in depression, reduced cognitive function, and decreased libido for both genders, as well as menstrual irregularities. The condition can be a direct result of blunt trauma to the pituitary. As well as hemorrhage, edema, shear injuries, and compromised blood flow as well as neuroinflammation.
 

Routine blood tests are not adequate to document hypopituitarism.

Unless specifically ordered, the insulin tolerance test (ITT) is not administered. The ITT involves a injection of insulin. Followed by several interval based blood glucose measurements. The ITT is the gold standard for assessing growth hormone deficiency. It may also hold the key to diagnosis following head injury.
 
For more on injury to the pituitary gland in traumatic brain injury; see Hacioglu A, Kelestimur F, Tanriverdi F., “Pituitary dysfunction due to sports-related traumatic brain injury”, Pituitary, 2019 Jan 14. (abstract only but complete article in possession of author).