A 42-year-old male construction worker comes to the primary care clinic for evaluation of … — USMLE Step 2 CK

A $42$ -year-old male construction worker comes to the primary care clinic for evaluation of profound fatigue, a $9$ -kg ( $20$ -lb) weight gain, and diminished libido progressing over the last nine months. He has a history of chronic, severe lumbar radiculopathy following a workplace injury four years ago. His daily medications include extended-release oxycodone, duloxetine, and ibuprofen. Vital signs show a blood pressure of $118/76$ mmHg and a heart rate of $58$ /min. Physical examination reveals a flat affect, dry skin, and a delayed relaxation phase of the Achilles tendon reflexes. Visual field testing is entirely normal. A comprehensive metabolic panel is unremarkable. Neuroimaging of the sella turcica with gadolinium reveals no masses or structural lesions.

Relevant laboratory results are as follows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.6 \text{ }\mu\text{U/mL}$	$0.5 - 5.0 \text{ }\mu\text{U/mL}$
Free thyroxine (T4)	$0.4 \text{ ng/dL}$	$0.9 - 2.4 \text{ ng/dL}$
Total testosterone (morning)	$120 \text{ ng/dL}$	$300 - 1000 \text{ ng/dL}$
Luteinizing hormone (LH)	$1.2 \text{ mIU/mL}$	$1.8 - 8.6 \text{ mIU/mL}$
Prolactin	$14 \text{ ng/mL}$	$< 20 \text{ ng/mL}$

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

Suppression of endogenous hypothalamic releasing factorsAnswer
B
Primary antibody-mediated failure of peripheral endocrine organs
C
Anatomical disruption of the hypothalamic-hypophyseal portal system
D
Decreased peripheral conversion of thyroxine to triiodothyronine
E
Post-translational structural mutation of hormone receptors

Answer

The mechanism is the suppression of endogenous hypothalamic releasing factors, leading to a drug-induced central hypogonadism and hypothyroidism.

The correct option identifies the pathophysiology of opioid-induced endocrinopathy. Chronic administration of opioids (like extended-release oxycodone) activates mu-opioid receptors in the hypothalamus. This activation strongly inhibits the pulsatile secretion of Gonadotropin-Releasing Hormone (GnRH) and, to a lesser extent, Thyrotropin-Releasing Hormone (TRH) and Corticotropin-Releasing Hormone (CRH). Consequently, the anterior pituitary is under-stimulated, leading to inappropriately normal or low levels of LH, FSH, and TSH, which subsequently causes low testosterone and low free T4. The normal prolactin and normal MRI rule out a compressive pituitary macroadenoma.

Step-by-Step Solution

Interpret the thyroid function tests.

Free T4 is low (

0.4 \text{ ng/dL}

), but TSH is inappropriately normal (

1.6 \text{ }\mu\text{U/mL}

Identifying that the low T4 is not accompanied by a high TSH confirms secondary (central) hypothyroidism rather than primary thyroid failure.

Interpret the gonadal function tests.

Total testosterone is low (

120 \text{ ng/dL}

), but LH is inappropriately normal (

1.2 \text{ mIU/mL}

Similar to the thyroid axis, a low testosterone with a non-elevated LH points to secondary (central) hypogonadism.

Synthesize the findings and identify the etiology based on the patient's history.

The patient has pan-central endocrine suppression with normal prolactin and normal neuroimaging. The use of chronic high-dose opioids (oxycodone) is the unifying cause.

Opioids directly inhibit the hypothalamus, decreasing the pulsatile release of GnRH and TRH, causing downstream suppression of both axes without altering pituitary structure or prolactin levels.

Key Concept

Opioid-induced endocrinopathy causes central hypogonadism and central hypothyroidism by suppressing the release of hypothalamic peptides (GnRH, TRH).

Estimated Time:1m 30s

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