Examkin

Question

Difficulty: MediumEndocrinology and Metabolism

A 48-year-old man comes to the clinic reporting progressive fatigue, muscle weakness, and decreased libido over the past 6 months. He has gained 4 kg (8.8 lb) during this period. The patient has a history of a traumatic brain injury sustained in a motor vehicle collision one year ago, which required a prolonged hospital stay. His vital signs are temperature 36.4°C (97.5°F), blood pressure 105/65 mmHg, and pulse 56/min. Physical examination shows loss of axillary and pubic hair, and delayed relaxation of deep tendon reflexes. Laboratory results are as follows:

Serum sodium: 132 mEq/L
Serum potassium: 4.0 mEq/L
Thyroid-stimulating hormone (TSH): 1.1 µU/mL (Normal: 0.4–4.0 µU/mL)
Free thyroxine (T4): 0.5 ng/dL (Normal: 0.7–1.9 ng/dL)

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

  1. Decreased secretion of thyrotropinAnswer
  2. B
    Autoimmune destruction of thyroid parenchyma
  3. C
    Inadequate production of thyroxine-binding globulin
  4. D
    Peripheral resistance to circulating thyroid hormones
  5. E
    Reduced peripheral deiodination of thyroxine

Answer

The correct answer is decreased secretion of thyrotropin, which indicates central (secondary) hypothyroidism.
The patient's clinical presentation is highly suggestive of hypothyroidism, which is confirmed by a low free T4 level. However, his TSH is within the normal reference range. In a patient with an intact pituitary gland, a low free T4 should trigger a compensatory rise in TSH (typically > 10 µU/mL). An inappropriately normal or low TSH in the setting of low free T4 defines central (secondary) hypothyroidism. This is caused by decreased secretion of thyrotropin (TSH) from the anterior pituitary, which in this patient is likely a late complication of his severe traumatic brain injury. The concomitant loss of axillary and pubic hair, decreased libido, and mild hyponatremia suggest broader panhypopituitarism affecting gonadotropins and possibly ACTH as well.

Step-by-Step Solution

1
Analyze the clinical presentation.
The patient has signs and symptoms of hypothyroidism (fatigue, weight gain, bradycardia, delayed reflexes) and hypogonadism (decreased libido, loss of body hair).
To identify the clinical syndrome before interpreting laboratory values.
2
Interpret the thyroid function tests.
Free T4 is clearly low (0.5 ng/dL), confirming hypothyroidism, but TSH is strictly within the normal range (1.1 µU/mL).
To differentiate between primary and central causes of thyroid dysfunction based on the feedback loop.
3
Synthesize the findings with the patient's medical history.
The combination of low free T4 and an inappropriately normal TSH points to a pituitary or hypothalamic defect (central hypothyroidism), likely a sequela of the severe traumatic brain injury he suffered one year ago.
To determine the exact underlying pathophysiologic mechanism.

Key Concept

Central (secondary) hypothyroidism is characterized by low free T4 levels with inappropriately normal or low TSH levels, and is often caused by pituitary adenomas, brain trauma, or infiltrative diseases.
Rate this question

Topics

Endocrinology and Metabolism

More questions from this topic