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Difficulty: EasyEndocrinology and Metabolism

A 4242-year-old man visits the outpatient physician reporting a 44-month history of profound tiredness, reduced sexual desire, and frequent morning headaches. Over the last several weeks, he has experienced near-miss accidents while driving because he did not see cars in the adjacent lanes. Vital signs are within normal limits. Laboratory studies reveal:

* Thyroid-stimulating hormone (TSH): 1.11.1 mU/L (Normal: 0.50.5 - 5.05.0 mU/L)
* Free thyroxine (T4): 0.40.4 ng/dL (Normal: 0.90.9 - 1.71.7 ng/dL)

Which of the following is the most likely underlying cause of this patient's presentation?

  1. A
    Lymphocytic infiltration of the thyroid parenchyma
  2. B
    Reduced systemic deiodination of tetraiodothyronine
  3. Compression of anterior pituitary cells by an expanding lesionAnswer
  4. D
    Target tissue unresponsiveness to circulating thyroid hormones
  5. E
    Synthesis of immunoglobulins that stimulate follicular receptors

Answer

Compression of anterior pituitary cells by an expanding lesion
The patient presents with classic signs of a pituitary macroadenoma, including mass effect (morning headaches and bitemporal hemianopsia leading to driving accidents) and hypopituitarism (decreased libido from secondary hypogonadism). The laboratory findings confirm central hypothyroidism, characterized by a low free T4 and an inappropriately normal TSH. A structurally normal pituitary gland would respond to a low free T4 by drastically increasing TSH secretion. The failure to do so indicates central pathology, most likely an expanding sellar lesion compressing the normal thyrotroph cells.

Step-by-Step Solution

1
Analyze the patient's clinical symptoms.
The patient has symptoms of hypothyroidism (fatigue), hypogonadism (decreased sexual desire), and a sellar mass effect (morning headaches, bitemporal hemianopsia affecting driving).
To identify the clinical syndrome pointing to the anatomic location of the pathology.
2
Interpret the thyroid function laboratory results.
The free T4 is distinctly low (0.40.4 ng/dL), but the TSH is normal (1.11.1 mU/L) rather than elevated.
To differentiate between primary thyroid failure and central (secondary) hypothyroidism.
3
Correlate laboratory findings with the clinical presentation.
The inappropriately normal TSH in the setting of low free T4 confirms central hypothyroidism. This aligns perfectly with the mass effect symptoms caused by a pituitary macroadenoma compressing both the optic chiasm and the normal anterior pituitary tissue.
To synthesize all data points and determine the underlying pathophysiologic mechanism.

Key Concept

Central hypothyroidism presents with a low free T4 and an inappropriately normal or low TSH, often accompanied by other signs of hypopituitarism and mass effect if caused by a pituitary adenoma.
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Topics

Endocrinology and Metabolism

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