Examkin

Question

Difficulty: EasyEndocrinology and Metabolism

An endocrinology consultation is requested for a 48-year-old male who has been experiencing progressive sluggishness, unexplained weight gain, and persistent chilliness for the past six months. Upon further questioning, he reports bothersome headaches and difficulty maintaining an erection. He has no significant past medical history and takes no medications. Vital signs are within normal limits, except for a pulse of 54/min54/\text{min}. Physical examination reveals dry, coarse skin and a notable delay in the relaxation phase of the Achilles tendon reflex. A confrontation visual field test detects bilateral peripheral vision deficits.

Laboratory evaluation reveals:
Serum thyroid-stimulating hormone (TSH): 1.4 mIU/L1.4 \text{ mIU/L} (normal: 0.5−5.0 mIU/L0.5-5.0 \text{ mIU/L})
Serum free thyroxine (T4): 0.4 ng/dL0.4 \text{ ng/dL} (normal: 0.9−1.7 ng/dL0.9-1.7 \text{ ng/dL})

Which of the following is the most likely etiology of this patient's thyroid laboratory abnormalities?

  1. A
    Decreased peripheral deiodination of thyroxine
  2. B
    Ectopic production of thyroid hormone
  3. Mass effect from a sellar lesionAnswer
  4. D
    Reduced production of thyroxine-binding globulin
  5. E
    Autoimmune infiltration of the thyroid gland

Answer

Mass effect from a sellar lesion
This patient presents with clinical signs of hypothyroidism and hypogonadism, alongside headaches and bitemporal hemianopsia (bilateral peripheral vision deficits). His laboratory evaluation confirms central (secondary) hypothyroidism, characterized by a low free T4 level with an inappropriately normal TSH. In a healthy hypothalamic-pituitary-thyroid axis, a low free T4 should trigger a robust elevation in TSH. The failure to do so, combined with visual field deficits, strongly indicates a space-occupying lesion in the sella turcica (such as a pituitary macroadenoma) causing mass effect and compressing the normal pituitary tissue and optic chiasm.

Step-by-Step Solution

1
Interpret the clinical symptoms and physical exam findings.
The patient has signs of hypothyroidism (fatigue, cold intolerance, weight gain, bradycardia, delayed reflexes), hypogonadism (erectile dysfunction), and a pituitary mass effect (headaches, bitemporal hemianopsia).
Identifying the clinical syndrome guides the interpretation of the laboratory values.
2
Analyze the thyroid laboratory panel.
Free T4 is overtly low, confirming hypothyroidism, but the TSH is inappropriately normal rather than elevated.
The relationship between free T4 and TSH distinguishes between primary and central causes of hypothyroidism.
3
Synthesize clinical and laboratory data to determine the etiology.
Central hypothyroidism combined with hypogonadism and visual field deficits is characteristic of a pituitary macroadenoma compressing the pituitary gland.
A sellar mass explains both the hormonal deficiencies and the compressive neurologic symptoms.

Key Concept

Central (secondary) hypothyroidism is characterized by a low serum free T4 level with an inappropriately normal or low TSH level, often caused by a compressive pituitary adenoma.
Rate this question

Topics

Endocrinology and Metabolism

More questions from this topic