A 35-year-old man comes to the clinic accompanied by his wife due to severe fatigue, cold … — USMLE Step 2 CK

A $35$ -year-old man comes to the clinic accompanied by his wife due to severe fatigue, cold intolerance, and weight gain over the past $4$ months. He also reports decreased libido and difficulty sustaining erections. His medical history is significant for a severe traumatic brain injury sustained in a motor vehicle collision $6$ months ago, which resulted in a diffuse axonal injury and a $2$ -week stay in the intensive care unit. His only medication is acetaminophen for occasional headaches. Vital signs show a temperature of $36.1^\circ$ C ( $97.0^\circ$ F), blood pressure of $102/64$ mmHg, and a pulse of $54$ /min. Physical examination reveals dry, pale skin, loss of axillary hair, and delayed relaxation of the deep tendon reflexes.

Laboratory results are as follows:

Test	Result	Reference Range
Serum sodium	$131$ mEq/L	$136$ - $145$ mEq/L
Serum potassium	$4.2$ mEq/L	$3.5$ - $5.0$ mEq/L
Thyroid-stimulating hormone (TSH)	$1.4$ mU/L	$0.4$ - $4.2$ mU/L
Free thyroxine (T4)	$0.5$ ng/dL	$0.9$ - $1.7$ ng/dL

Based on the patient's current condition, which of the following additional laboratory findings is most likely present?

A
Elevated anti-thyroid peroxidase antibodies
B
Elevated serum luteinizing hormone (LH)
Decreased serum morning cortisolAnswer
D
Elevated serum adrenocorticotropic hormone (ACTH)
E
Decreased serum aldosterone

Answer

Decreased serum morning cortisol

The correct answer is decreased serum morning cortisol. This patient presents with fatigue, weight gain, cold intolerance, and hypogonadism. His laboratory results show a low Free T4 (

0.5

ng/dL) with an inappropriately normal TSH (

1.4

mU/L), establishing the diagnosis of central (secondary) hypothyroidism. Given his history of a severe traumatic brain injury (TBI), these findings are highly indicative of post-traumatic panhypopituitarism. TBI can cause damage to the pituitary gland or infundibular stalk through shear forces, direct trauma, or vascular compromise. Panhypopituitarism involves the deficiency of multiple anterior pituitary hormones. Along with TSH and gonadotropins (LH/FSH), adrenocorticotropic hormone (ACTH) production is impaired. The lack of ACTH stimulation leads to central adrenal insufficiency, which manifests as a decreased serum morning cortisol level. The patient's euvolemic hyponatremia (

131

mEq/L) with normal potassium is also characteristic of isolated cortisol deficiency, as cortisol normally inhibits antidiuretic hormone (ADH) release.

Step-by-Step Solution

Analyze the patient's history and symptoms

The patient has a history of severe traumatic brain injury (TBI) 6 months ago and now presents with symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, delayed reflexes) and hypogonadism (decreased libido, loss of axillary hair).

To identify potential systemic etiologies for his multiple endocrine complaints.

Interpret the thyroid function tests

The Free T4 is low (

0.5

ng/dL) with an inappropriately normal TSH (

1.4

mU/L).

To classify the hypothyroidism as primary (thyroid origin) or central/secondary (pituitary/hypothalamic origin).

Synthesize findings to establish a unifying diagnosis

The combination of central hypothyroidism, symptoms of hypogonadism, and a severe prior TBI points directly to post-traumatic panhypopituitarism.

TBI is a well-known cause of anterior pituitary dysfunction due to stalk shearing or vascular compromise.

Predict other expected laboratory abnormalities

Along with TSH and gonadotropins, ACTH production is likely impaired, which would lead to central adrenal insufficiency and decreased morning cortisol.

Panhypopituitarism typically involves multiple anterior pituitary axes, and recognizing concomitant adrenal insufficiency is critical for safe management.

Key Concept

Post-traumatic panhypopituitarism presenting as central hypothyroidism and central adrenal insufficiency

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic