Examkin

Question

Difficulty: MediumGeneral Internal Medicine

A 38-year-old man presents to the clinic complaining of progressive fatigue, generalized weakness, and cold intolerance over the past 8 months. He also reports a significantly decreased libido. Two years ago, he was involved in a severe motor vehicle collision that resulted in a basilar skull fracture and a prolonged intensive care unit stay. He takes no daily medications. His temperature is 36.4∘C36.4^\circ\text{C} (97.5∘F97.5^\circ\text{F}), pulse is 55/min, and blood pressure is 102/64 mmHg. Physical examination demonstrates pale, dry skin and a delayed relaxation phase of the ankle reflexes. Visual field testing is normal.

Laboratory results are as follows:

Laboratory TestResultReference Range
Thyroid-stimulating hormone (TSH)1.2 \muU/mL1.2 \text{ \mu U/mL}0.5−5.0 \muU/mL0.5-5.0 \text{ \mu U/mL}
Free thyroxine (T4)0.4 ng/dL0.4 \text{ ng/dL}0.9−1.7 ng/dL0.9-1.7 \text{ ng/dL}
Total testosterone140 ng/dL140 \text{ ng/dL}300−1000 ng/dL300-1000 \text{ ng/dL}
Luteinizing hormone (LH)1.1 mIU/mL1.1 \text{ mIU/mL}2.0−12.0 mIU/mL2.0-12.0 \text{ mIU/mL}

Which of the following is the most likely underlying mechanism for this patient's condition?

  1. A
    Autoimmune destruction of thyroid follicular cells
  2. B
    Primary failure of testicular Leydig cells
  3. Disruption of the hypothalamic-pituitary axisAnswer
  4. D
    Peripheral tissue resistance to thyroid hormone
  5. E
    Autonomous production of thyroid hormone

Answer

Disruption of the hypothalamic-pituitary axis
The correct answer identifies that the patient's combined laboratory findings and history point to a central endocrine defect. The patient has a low Free T4, but his TSH is not elevated (it is inappropriately normal). This indicates a failure of the pituitary to respond to low circulating thyroid hormone levels, characteristic of secondary hypothyroidism. Similarly, his testosterone is low without an appropriate compensatory rise in LH, indicating secondary hypogonadism. Given his history of severe traumatic brain injury and basilar skull fracture, the most likely mechanism is disruption of the hypothalamic-pituitary axis (traumatic hypopituitarism), leading to multiple anterior pituitary hormone deficiencies.

Step-by-Step Solution

1
Analyze the patient's presenting symptoms and physical exam findings.
The patient exhibits classic signs of hypothyroidism (fatigue, cold intolerance, delayed reflexes) and hypogonadism (decreased libido).
Identifying the clinical syndromes guides the interpretation of the subsequent laboratory data.
2
Interpret the thyroid function tests.
Free T4 is low, but TSH is inappropriately normal rather than elevated, indicating central (secondary) hypothyroidism.
To distinguish between primary thyroid failure and a central defect.
3
Interpret the gonadal function tests.
Testosterone is low, but LH is inappropriately normal rather than elevated, indicating central (secondary) hypogonadism.
To determine if the hypogonadism is due to primary testicular failure or a central defect.
4
Synthesize the laboratory findings with the patient's medical history.
The presence of multiple central endocrine deficiencies in a patient with a history of severe traumatic brain injury points to post-traumatic hypopituitarism.
To determine the most likely underlying pathophysiologic mechanism.

Key Concept

Secondary (central) hypothyroidism and hypogonadism caused by traumatic brain injury (post-traumatic hypopituitarism).
Rate this question

Topics

General Internal Medicine

More questions from this topic