A 65-year-old man is evaluated during a routine follow-up appointment. He reports feeling … — USMLE Step 2 CK

A 65-year-old man is evaluated during a routine follow-up appointment. He reports feeling unusually tired over the past few months, along with new-onset constipation and dry skin. His medical history is notable for the transsphenoidal resection of a nonfunctioning pituitary macroadenoma two years ago. He currently takes no medications. Vital signs are normal except for a heart rate of 58/min. Physical examination reveals delayed relaxation of the deep tendon reflexes. Laboratory studies show a thyroid-stimulating hormone (TSH) level of $1.8 \text{ \mu U/mL}$ (normal: $0.5\text{-}5.0 \text{ \mu U/mL}$ ) and a free T4 level of $0.5 \text{ ng/dL}$ (normal: $0.9\text{-}1.7 \text{ ng/dL}$ ). Which of the following is the most likely diagnosis?

Central hypothyroidismAnswer
B
Primary hypothyroidism
C
Subclinical hypothyroidism
D
Euthyroid sick syndrome
E
Thyroid hormone resistance

Answer

Central hypothyroidism

The correct diagnosis is central (secondary) hypothyroidism. The patient has clear clinical features of hypothyroidism, supported by a distinctly low free T4 level. In a normal hypothalamic-pituitary-thyroid axis, a low free T4 removes negative feedback and triggers a compensatory, significant increase in TSH (which would indicate primary hypothyroidism). In this patient, the TSH is inappropriately normal, indicating a failure of the pituitary gland to respond to the low circulating thyroxine levels. This laboratory pattern, combined with his surgical history of a transsphenoidal pituitary macroadenoma resection, firmly establishes the diagnosis of central hypothyroidism.

Step-by-Step Solution

Identify the patient's clinical syndrome.

The patient has signs and symptoms of hypothyroidism, including fatigue, constipation, dry skin, bradycardia, and delayed deep tendon reflexes.

Recognizing the clinical presentation guides the interpretation of the subsequent laboratory values.

Analyze the thyroid laboratory results.

The free T4 level is low (

0.5 \text{ ng/dL}

), confirming a hypothyroid state. The TSH level is normal (

1.8 \text{ \mu U/mL}

Evaluating the relationship between TSH and free T4 is essential for determining the etiology of the thyroid dysfunction.

Determine the etiology based on the laboratory pattern and patient history.

A low free T4 should physiologically trigger an elevated TSH. An inappropriately normal TSH indicates pituitary failure, which aligns with his history of pituitary adenoma resection.

Integrating lab patterns with surgical history confirms the specific diagnosis of central (secondary) hypothyroidism.

Key Concept

Diagnosis of Central Hypothyroidism

Estimated Time:45s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic