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Difficulty: MediumEndocrinology and Metabolism

A 29-year-old man comes to the clinic for a follow-up examination. Over the past 4 months, he has experienced profound fatigue, cold intolerance, a 6-kg (13.2-lb) weight gain, and decreased libido. Eight months ago, he was involved in a severe motorcycle collision that resulted in a traumatic brain injury. He spent 3 weeks in the intensive care unit but has since been participating in a physical rehabilitation program. His current medications include ibuprofen as needed for occasional headache. Blood pressure is 106/68 mm Hg and pulse is 56/min. Physical examination reveals dry skin, loss of axillary and pubic hair, and a delayed relaxation phase of the deep tendon reflexes. Laboratory studies show:

Laboratory StudyResultReference Range
Thyroid-stimulating hormone (TSH)1.1 µU/mL0.4 - 4.0 µU/mL
Free thyroxine (T4)0.5 ng/dL0.9 - 2.4 ng/dL
Luteinizing hormone (LH)1.2 mIU/mL1.5 - 9.3 mIU/mL
Testosterone140 ng/dL300 - 1000 ng/dL

Which of the following is the most likely explanation for this patient's clinical and laboratory findings?

  1. A
    Autoimmune destruction of the thyroid parenchyma
  2. B
    Decreased peripheral conversion of thyroxine to triiodothyronine
  3. Disruption of the hypothalamic-pituitary axisAnswer
  4. D
    End-organ resistance to thyroid hormones
  5. E
    Primary failure of the testicular Leydig cells

Answer

Disruption of the hypothalamic-pituitary axis
The patient's clinical presentation (fatigue, cold intolerance, weight gain, bradycardia, delayed deep tendon reflexes) and laboratory findings (low free T4 with inappropriately normal TSH) confirm central (secondary) hypothyroidism. Furthermore, he exhibits central hypogonadism (low testosterone with an inappropriately normal LH) and physical signs of other hormone deficiencies (loss of axillary/pubic hair). The onset of these symptoms following a severe traumatic brain injury (TBI) strongly suggests panhypopituitarism caused by disruption of the hypothalamic-pituitary axis. TBI can cause shearing forces on the infundibulum (pituitary stalk), microvascular ischemic insult, or direct trauma to the pituitary gland, resulting in single or multiple anterior pituitary hormone deficiencies.

Step-by-Step Solution

1
Analyze the patient's thyroid laboratory results.
Low free T4 (0.5 ng/dL) with an inappropriately normal TSH (1.1 µU/mL).
To identify the presence and specific etiology (primary vs. central) of hypothyroidism.
2
Analyze the patient's gonadal laboratory results.
Low testosterone (140 ng/dL) with an inappropriately normal/low LH (1.2 mIU/mL).
To evaluate the cause of decreased libido and assess for broader pituitary dysfunction.
3
Synthesize the laboratory findings with the patient's clinical history.
The patient has combined central hypothyroidism and central hypogonadism following a severe traumatic brain injury.
To establish the underlying diagnosis of post-traumatic hypopituitarism.

Key Concept

Traumatic brain injury is a notable cause of acquired hypopituitarism due to shearing forces on the pituitary stalk or vascular injury. It presents with deficiencies in target-organ hormones (e.g., T4, testosterone) alongside inappropriately normal or low levels of pituitary trophic hormones (e.g., TSH, LH).
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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