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Difficulty: MediumEndocrinology and Metabolism

A 54-year-old man presents to the clinic reporting a 4-month history of persistent fatigue, sluggishness, and increased sensitivity to cold. He also mentions a recent decline in his sexual drive and difficulty maintaining an erection. He has no past medical history and takes no medications. His temperature is 36.2∘C36.2^{\circ}\text{C} (97.2∘F97.2^{\circ}\text{F}), blood pressure is 108/68 mm Hg108/68\text{ mm Hg}, and pulse is 55/min55\text{/min}. Physical examination demonstrates pale, dry skin and a noticeable delay in the relaxation phase of the patellar reflexes. Confrontational visual field testing detects a deficit in the temporal fields bilaterally.

Laboratory results are as follows:
Thyroid-stimulating hormone (TSH): 1.8 μIU/mL1.8\text{ }\mu\text{IU/mL} (normal: 0.4−4.0 μIU/mL0.4-4.0\text{ }\mu\text{IU/mL})
Free thyroxine (T4): 0.5 ng/dL0.5\text{ ng/dL} (normal: 0.9−1.7 ng/dL0.9-1.7\text{ ng/dL})
Total testosterone: 140 ng/dL140\text{ ng/dL} (normal: 300−1,000 ng/dL300-1,000\text{ ng/dL})

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

  1. Mass effect compressing the normal tissue within the sella turcicaAnswer
  2. B
    Chronic lymphocytic infiltration of the thyroid stroma
  3. C
    Downregulation of peripheral 5'-deiodinase activity
  4. D
    Target tissue resistance to circulating thyroid hormones
  5. E
    Excessive production of biologically inactive thyroxine-binding globulin

Answer

Mass effect compressing the normal tissue within the sella turcica
The correct answer identifies a compressive pituitary mass (such as a nonfunctioning macroadenoma). The patient's clinical presentation includes symptoms of hypothyroidism (fatigue, cold intolerance, delayed reflexes) and hypogonadism (decreased libido, erectile dysfunction). Laboratory testing confirms low free T4 and low testosterone. Crucially, the TSH is inappropriately normal rather than elevated. In the setting of low target organ hormones, a lack of compensatory elevation in pituitary tropic hormones (TSH, LH/FSH) indicates central (secondary) failure. The combination of central hypothyroidism, central hypogonadism, and bitemporal hemianopsia (temporal visual field deficits) strongly suggests a mass in the sella turcica compressing both the normal anterior pituitary tissue and the overlying optic chiasm.

Step-by-Step Solution

1
Analyze the patient's symptoms and physical examination findings.
The patient presents with symptoms of hypothyroidism (fatigue, cold intolerance, delayed reflexes), hypogonadism (decreased libido), and an optic chiasm lesion (bitemporal hemianopsia).
Identifying the clinical syndromes points toward a multiaxial endocrine defect combined with a structural neurologic deficit.
2
Evaluate the laboratory results, particularly the relationship between TSH and free T4.
Free T4 is low (0.5 ng/dL0.5\text{ ng/dL}), but TSH is inappropriately normal (1.8 μIU/mL1.8\text{ }\mu\text{IU/mL}). Testosterone is also low.
A low free T4 with a normal TSH indicates central (secondary) hypothyroidism due to pituitary or hypothalamic dysfunction, distinguishing it from primary thyroid failure.
3
Synthesize the laboratory data with the physical exam findings to determine the underlying pathology.
The combination of central hypothyroidism, central hypogonadism, and bitemporal hemianopsia strongly indicates a compressive mass in the sella turcica, such as a pituitary macroadenoma.
A large pituitary mass can compress the optic chiasm above it and the normal anterior pituitary tissue around it, resulting in panhypopituitarism and visual field defects.

Key Concept

Diagnosis of central (secondary) hypothyroidism and panhypopituitarism due to a compressive sellar mass.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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