A 28-year-old man is evaluated in the outpatient clinic for profound fatigue, an 8-kg (17.6-lb) weight gain, and decreased libido that have worsened over the past six months. One year ago, he sustained a severe traumatic brain injury in a motorcycle collision, which required a three-week stay in the intensive care unit. He is currently taking no medications. Vital signs are within normal limits. Physical examination reveals a flat affect, delayed relaxation of deep tendon reflexes, and sparse axillary and pubic hair. Laboratory studies show:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.1 µU/mL | 0.4–4.0 µU/mL |
| Free thyroxine (T4) | 0.5 ng/dL | 0.9–1.7 ng/dL |
| Total testosterone | 150 ng/dL | 300–1000 ng/dL |
Which of the following is the most likely underlying mechanism for this patient's thyroid laboratory findings?
- Insufficient secretion of thyrotropinAnswer
- BAutoimmune destruction of thyroid follicles
- CReduced peripheral deiodination of thyroxine
- DImpaired synthesis of thyroxine-binding globulin
- EInadequate dietary intake of iodine
Answer
Insufficient secretion of thyrotropin
This patient's clinical presentation and laboratory findings are highly consistent with panhypopituitarism secondary to a prior severe traumatic brain injury (TBI). Severe head trauma can cause shearing forces on the pituitary stalk or ischemic damage to the pituitary gland, leading to deficiencies in multiple anterior pituitary hormones. The laboratory pattern of a low free T4 paired with an inappropriately normal TSH indicates central (secondary) hypothyroidism, which is caused by insufficient secretion of thyrotropin (TSH) from the anterior pituitary. This central etiology is further supported by evidence of a second pituitary hormone deficiency, manifested as secondary hypogonadism (low testosterone).
Step-by-Step Solution
Key Concept
Diagnosis of central (secondary) hypothyroidism