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Difficulty: MediumEndocrinology and Metabolism

A 42-year-old man presents to his primary care provider reporting a gradual onset of profound sluggishness, increased sensitivity to cold, and worsening constipation over the past five months. He also mentions a decreased interest in sexual activity. He has no significant past medical history and takes no medications. Vital signs show a temperature of 36.4°C (97.5°F), blood pressure of 105/70 mm Hg, and a pulse of 54/min. Physical examination reveals a puffy facial appearance, dry skin, and a delayed relaxation phase of the deep tendon reflexes. Visual field testing is intact to confrontation. Laboratory evaluation yields the following results:

Laboratory TestResultReference Range
Thyroid-stimulating hormone (TSH)2.1 mU/L0.4-4.2 mU/L
Free thyroxine (T4)0.4 ng/dL0.9-1.7 ng/dL

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

  1. Insufficient secretion of thyrotropinAnswer
  2. B
    Autoimmune destruction of thyroid follicular cells
  3. C
    Reduced peripheral conversion of thyroxine to triiodothyronine
  4. D
    Receptor-mediated tissue resistance to thyroid hormone
  5. E
    Dietary deficiency impairing thyroid hormone synthesis

Answer

Insufficient secretion of thyrotropin
This patient presents with classic signs and symptoms of hypothyroidism, including fatigue, cold intolerance, constipation, weight gain, bradycardia, and delayed relaxation of deep tendon reflexes. The laboratory results confirm the hypothyroid state with a low free T4 level. However, the TSH is within the normal reference range (2.1 mU/L). In a healthy hypothalamic-pituitary-thyroid (HPT) axis, a low free T4 removes negative feedback on the anterior pituitary, which should result in a significantly elevated TSH (primary hypothyroidism). The presence of a low free T4 alongside an 'inappropriately normal' (or frankly low) TSH strongly suggests a failure of the anterior pituitary to secrete adequate thyrotropin (TSH). This is known as central or secondary hypothyroidism. The patient's concurrent decreased libido may also point to hypogonadotropic hypogonadism, further supporting a broader pituitary dysfunction (panhypopituitarism) which requires MRI imaging of the brain.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and vital signs.
The patient exhibits fatigue, cold intolerance, constipation, bradycardia, and delayed deep tendon reflexes, establishing a clear clinical picture of symptomatic hypothyroidism.
Identifying the clinical state determines whether the subsequent laboratory values represent a pathological state or a benign variant.
2
Evaluate the laboratory thyroid panel in the context of the clinical state.
Free T4 is overtly low (0.4 ng/dL), confirming the hypothyroid state. However, the TSH is within the normal reference range (2.1 mU/L) rather than being elevated.
The relationship between T4 and TSH localizes the defect within the hypothalamic-pituitary-thyroid (HPT) axis.
3
Synthesize findings to determine the underlying mechanism.
A low free T4 with an inappropriately normal TSH indicates a failure of the pituitary gland to mount a compensatory TSH response. This defines central (secondary) hypothyroidism, caused by insufficient secretion of thyrotropin.
A primary thyroid defect would lack negative feedback, driving TSH levels above the reference range.

Key Concept

In central (secondary) hypothyroidism, patients present with clinical features of hypothyroidism and a low free T4, but the expected compensatory rise in TSH is absent (TSH remains inappropriately normal or low).
Estimated Time:1m 0s
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Topics

Endocrinology and Metabolism

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