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Zorluk: OrtaEndocrinology and Metabolism

During a routine primary care visit, a 4545-year-old man reports a 55-month history of worsening fatigue, generalized weakness, and decreased libido. He takes no daily medications and his past medical history is unremarkable. On physical examination, his heart rate is 5454/min and blood pressure is 112/70112/70 mmHg. His skin is notably dry, and deep tendon reflexes exhibit a delayed relaxation phase. Visual field testing by confrontation reveals no deficits. Comprehensive metabolic panel is normal. Thyroid function testing reveals a free thyroxine (T4) concentration of 0.50.5 ng/dL (Reference: 0.9−1.70.9-1.7 ng/dL) and a thyroid-stimulating hormone (TSH) concentration of 2.42.4 mIU/L (Reference: 0.4−4.00.4-4.0 mIU/L). Which of the following is the most appropriate next step in the evaluation of this patient?

  1. Measurement of 8 AM serum cortisolCevap
  2. B
    Measurement of thyroid peroxidase antibodies
  3. C
    Initiation of levothyroxine therapy
  4. D
    Ultrasound of the thyroid gland
  5. E
    Measurement of serum reverse triiodothyronine

Cevap

Measurement of 8 AM serum cortisol
The correct answer is to measure an 8 AM serum cortisol. This patient has clinical signs of hypothyroidism (fatigue, bradycardia, delayed deep tendon reflexes) confirmed by a low free T4 level. However, the TSH level is inappropriately normal. In a healthy pituitary gland, a low free T4 would result in significantly elevated TSH due to the loss of negative feedback. The combination of low free T4 and normal (or low) TSH strongly suggests central hypothyroidism. Whenever central hypothyroidism is suspected, it is critical to evaluate for other pituitary hormone deficiencies. Most importantly, the hypothalamic-pituitary-adrenal (HPA) axis must be assessed (usually starting with an 8 AM serum cortisol) before initiating levothyroxine. Thyroid hormone increases the hepatic clearance of cortisol; administering levothyroxine to a patient with undiagnosed concomitant central adrenal insufficiency can rapidly precipitate a life-threatening adrenal crisis.

Adım Adım Çözüm

1
Interpret the laboratory findings.
The patient has a low free T4 with a TSH that is inappropriately within the normal range.
In primary hypothyroidism, a low T4 should stimulate the pituitary to produce high levels of TSH. A normal or low TSH indicates a central (pituitary or hypothalamic) etiology.
2
Identify the underlying diagnosis.
The laboratory pattern is diagnostic of central (secondary) hypothyroidism.
The pituitary gland is failing to mount an appropriate TSH response to low circulating thyroid hormone.
3
Determine the next best step in management.
Evaluate for other pituitary hormone deficiencies, specifically central adrenal insufficiency, by measuring an 8 AM serum cortisol.
Starting thyroid hormone replacement increases the metabolic clearance of cortisol. If a patient has undiagnosed concomitant central adrenal insufficiency, initiating levothyroxine can precipitate a fatal adrenal crisis.

Anahtar Kavram

Patients with central hypothyroidism (low free T4, inappropriately normal/low TSH) must be evaluated for central adrenal insufficiency prior to initiating levothyroxine to prevent an adrenal crisis.
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Endocrinology and Metabolism

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