A 29-year-old nulligravid woman comes to the clinic due to the absence of menstrual period… — USMLE Step 2 CK

A $29$ -year-old nulligravid woman comes to the clinic due to the absence of menstrual periods for the past $6$ months. Prior to this, she had regular $28$ -day cycles. Over the same timeframe, she has noted an unintentional $5$ -kg ( $11$ -lb) weight gain, generalized fatigue, and the new onset of coarse dark hair on her upper lip and chin. She is sexually active and uses condoms for contraception, though she and her partner wish to conceive in the next year. Her younger sister was recently diagnosed with polycystic ovary syndrome (PCOS). Vital signs include a blood pressure of $114/72$ mm Hg, a heart rate of $54$ /min, and a temperature of $36.6$ °C ( $97.9$ °F). Her body mass index is $31$ kg/m $^2$ . Physical examination reveals delayed relaxation of the deep tendon reflexes and moderate facial hirsutism. A urine pregnancy test is negative.

Laboratory results are as follows:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$15.4$ $\mu$ U/mL	$0.4-4.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9-1.7$ ng/dL
Prolactin	$48$ ng/mL	$<25$ ng/mL
Follicle-stimulating hormone (FSH)	$3$ mIU/mL	$4.7-21.5$ mIU/mL
Luteinizing hormone (LH)	$2$ mIU/mL	$5.0-25.0$ mIU/mL
Total testosterone	$65$ ng/dL	$15-70$ ng/dL

Which of the following is the most appropriate next step in the management of this patient?

Initiate levothyroxine therapyAnswer
B
Order magnetic resonance imaging of the brain
C
Recommend combined oral contraceptive pills
D
Perform an endometrial biopsy
E
Administer a medroxyprogesterone acetate challenge

Answer

The most appropriate next step is to initiate levothyroxine therapy to treat her underlying primary hypothyroidism.

The correct action is initiating levothyroxine therapy. This patient's secondary amenorrhea and hirsutism are direct consequences of primary hypothyroidism. In primary hypothyroidism, the lack of negative feedback from thyroid hormones causes the hypothalamus to secrete high levels of thyrotropin-releasing hormone (TRH). TRH stimulates the anterior pituitary to release both TSH and prolactin. The resulting hyperprolactinemia inhibits pulsatile gonadotropin-releasing hormone (GnRH) release, which decreases FSH and LH, leading to anovulation and secondary amenorrhea. Furthermore, hypothyroidism decreases the hepatic synthesis of sex hormone-binding globulin (SHBG). This results in a higher proportion of free (biologically active) testosterone, which causes hirsutism even when total testosterone levels remain in the normal or high-normal range. Treating the underlying hypothyroidism with levothyroxine will normalize TRH, which in turn resolves the hyperprolactinemia (restoring ovulatory cycles) and normalizes SHBG (improving hirsutism).

Step-by-Step Solution

Analyze the clinical presentation and physical examination.

The patient has secondary amenorrhea, fatigue, weight gain, bradycardia, delayed deep tendon reflexes, and new-onset hirsutism.

These findings are highly suggestive of a systemic endocrine disorder, specifically hypothyroidism, alongside a hyperandrogenic state.

Interpret the laboratory findings.

Markedly elevated TSH and low free T4 confirm primary hypothyroidism. Prolactin is mildly elevated. FSH and LH are low-normal. Total testosterone is high-normal.

Accurate laboratory interpretation is required to distinguish primary from secondary endocrine failure and rule out primary ovarian or pituitary pathologies.

Connect the physiological pathway linking primary hypothyroidism to amenorrhea and hirsutism.

Low T4 reduces negative feedback, increasing TRH. High TRH stimulates both TSH and prolactin. Elevated prolactin inhibits GnRH, leading to low FSH/LH and amenorrhea. Concurrently, hypothyroidism decreases hepatic production of sex hormone-binding globulin (SHBG), increasing free (active) testosterone, causing hirsutism despite normal total testosterone.

Understanding this cascade proves that all symptoms (including the PCOS-like features) originate from the thyroid dysfunction.

Determine the optimal management.

Initiate levothyroxine therapy.

Restoring a euthyroid state with levothyroxine will lower TRH, normalize prolactin (restoring menses), and increase SHBG (resolving hirsutism) without needing additional targeted therapies.

Key Concept

Primary hypothyroidism can cause secondary amenorrhea via TRH-induced hyperprolactinemia and hirsutism via decreased sex hormone-binding globulin (SHBG).

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