A 48-year-old man comes to the clinic reporting a 6-month history of worsening fatigue, ge… — USMLE Step 2 CK

A 48-year-old man comes to the clinic reporting a 6-month history of worsening fatigue, generalized joint pain, and decreased libido. His joint pain is most prominent in the second and third metacarpophalangeal joints of both hands. He has no significant past medical history and takes no medications. Vital signs are within normal limits. Physical examination reveals hepatomegaly and bilateral testicular atrophy. Skin examination shows a diffuse, slate-gray hyperpigmentation that is most pronounced in sun-exposed areas. Laboratory studies show the following:

Fasting blood glucose: $135 \text{ mg/dL}$
Thyroid-stimulating hormone (TSH): $1.4 \text{ mIU/L}$ (normal: $0.4–4.0 \text{ mIU/L}$ )
Free thyroxine (T4): $0.6 \text{ ng/dL}$ (normal: $0.9–1.7 \text{ ng/dL}$ )
Total testosterone: $140 \text{ ng/dL}$ (normal: $300–1,000 \text{ ng/dL}$ )
Luteinizing hormone (LH): $2.1 \text{ mIU/mL}$ (normal: $1.8–8.6 \text{ mIU/mL}$ )

Which of the following is the most likely mechanism underlying this patient's thyroid laboratory abnormalities?

Deposition of iron within the anterior pituitary glandAnswer
B
Autoimmune destruction of thyroid follicular cells
C
Deposition of iron within the thyroid gland parenchyma
D
Decreased hepatic synthesis of thyroxine-binding globulin
E
Reduced peripheral conversion of thyroxine to triiodothyronine

Answer

Deposition of iron within the anterior pituitary gland

This patient's clinical presentation (fatigue, hepatomegaly, slate-gray hyperpigmentation, arthropathy of the 2nd/3rd metacarpophalangeal joints, and elevated glucose) is classic for hereditary hemochromatosis. Endocrine dysfunction is a common complication of hemochromatosis, most frequently manifesting as hypogonadotropic hypogonadism and central hypothyroidism. This occurs because the anterior pituitary gland is highly susceptible to iron deposition, which damages the hormone-producing cells. The laboratory findings confirm central defects: the Free T4 is low, but the TSH is inappropriately normal rather than elevated. Similarly, the testosterone is low, but the LH is inappropriately normal. Both findings point directly to pituitary failure caused by iron deposition.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient exhibits classic signs of hereditary hemochromatosis: hyperpigmentation ('bronze diabetes'), hepatomegaly, arthropathy specifically affecting the 2nd and 3rd metacarpophalangeal joints, elevated fasting glucose, and hypogonadism.

Identifying the underlying systemic disease is necessary to explain the multi-organ involvement.

Interpret the thyroid laboratory profile.

The patient has a low Free T4 level accompanied by a TSH level that is within the normal range.

A low Free T4 should normally trigger an elevated TSH via negative feedback. An inappropriately normal or low TSH in the setting of low Free T4 indicates central (secondary) hypothyroidism.

Link the systemic disease to the specific endocrine pathology.

Hereditary hemochromatosis commonly causes iron deposition in the anterior pituitary gland, disrupting its ability to secrete trophic hormones like TSH and LH.

This explains the simultaneous occurrence of central hypothyroidism and central hypogonadism (low testosterone with normal/low LH) in this patient.

Key Concept

Hereditary hemochromatosis commonly causes central endocrinopathies (central hypothyroidism and central hypogonadism) due to iron deposition in the anterior pituitary gland.

Estimated Time:1m 30s

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