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Zorluk: KolayEndocrinology and Metabolism

A 50-year-old man visits his primary care provider due to worsening fatigue, sluggishness, and increased sensitivity to cold over the past 6 months. He has also noticed decreased libido and erectile dysfunction. His vital signs show a pulse of 54/min and blood pressure of 110/70 mm Hg. Physical examination reveals dry skin and delayed relaxation of the Achilles tendon reflexes. Visual field testing demonstrates decreased peripheral vision on the lateral sides of both eyes (bitemporal hemianopsia). Laboratory testing reveals a free thyroxine (T4) level of 0.4 ng/dL (normal: 0.9-1.7 ng/dL) and a thyroid-stimulating hormone (TSH) level of 1.1 µU/mL (normal: 0.5-5.0 µU/mL). Which of the following best explains this patient's thyroid function test results?

  1. Impaired production of thyrotropinCevap
  2. B
    Cell-mediated destruction of the thyroid parenchyma
  3. C
    Circulating antibodies stimulating the thyrotropin receptor
  4. D
    Reduced binding of thyroxine to carrier proteins
  5. E
    Tissue unresponsiveness to circulating thyroid hormones

Cevap

Impaired production of thyrotropin
The correct answer accurately identifies the pathophysiology of central hypothyroidism. This patient's fatigue, cold intolerance, dry skin, and delayed reflexes confirm clinical hypothyroidism, supported by a low free T4 level. In primary hypothyroidism (the most common cause), the intact pituitary gland senses the low T4 and secretes large amounts of TSH to stimulate the failing thyroid gland. In this patient, the TSH is inappropriately normal, indicating that the pituitary is failing to respond to the low thyroid hormone levels. This points to central hypothyroidism (impaired production of thyrotropin). The presence of bitemporal hemianopsia (due to optic chiasm compression) and symptoms of hypogonadism further support the diagnosis of a pituitary macroadenoma compressing normal anterior pituitary tissue.

Adım Adım Çözüm

1
Interpret the clinical presentation.
The patient exhibits classic symptoms of hypothyroidism (fatigue, cold intolerance, dry skin, bradycardia, delayed reflexes).
Identifying the clinical syndrome guides the interpretation of the laboratory values.
2
Analyze the thyroid function tests.
Free T4 is low, confirming hypothyroidism. TSH is inappropriately normal (1.1 µU/mL) rather than elevated.
An intact pituitary gland would mount a robust TSH response to a low free T4. The lack of this response localizes the pathology to the pituitary or hypothalamus.
3
Synthesize the laboratory findings with additional clinical clues.
Bitemporal hemianopsia and hypogonadism (decreased libido, erectile dysfunction) suggest a space-occupying lesion in the sella turcica (e.g., pituitary macroadenoma) causing panhypopituitarism.
This confirms the diagnosis of central (secondary) hypothyroidism due to impaired thyrotropin (TSH) production by the anterior pituitary.

Anahtar Kavram

Central (secondary) hypothyroidism is characterized by a low free T4 and an inappropriately normal or low TSH, often accompanied by other signs of pituitary dysfunction.
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Endocrinology and Metabolism

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