A 32-year-old female fitness instructor is evaluated for a 4-week history of heart palpita… — USMLE Step 3

A $32$ -year-old female fitness instructor is evaluated for a $4$ -week history of heart palpitations, severe anxiety, heat intolerance, and excessive sweating. She reports strict adherence to a new competition diet and taking various over-the-counter supplements for "cutting weight." Blood pressure is $145/88$ mm Hg, and pulse is $118$ /min. Physical examination shows a fine resting tremor in both hands and hyperreflexia. The thyroid gland is non-palpable and non-tender. There is no proptosis, periorbital edema, or pretibial myxedema.

Laboratory evaluation shows the following:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$< 0.01$ $\mu$ U/mL	$0.4 - 4.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.3$ ng/dL	$0.8 - 1.8$ ng/dL
Total triiodothyronine (T3)	$350$ ng/dL	$80 - 200$ ng/dL
Thyroglobulin	$< 0.5$ ng/mL	$3 - 40$ ng/mL
Thyrotropin receptor antibodies	Negative	Negative

Which of the following is the most likely diagnosis?

Exogenous triiodothyronine (T3) ingestionCevap
B
Exogenous levothyroxine (T4) ingestion
C
Subacute granulomatous thyroiditis
D
Struma ovarii
E
Graves disease

Cevap

Exogenous triiodothyronine (T3) ingestion

This patient presents with clinical thyrotoxicosis and a suppressed TSH, indicating primary hyperthyroidism or exogenous intake. The critical distinguishing laboratory findings are the paradoxically low free T4 alongside an elevated total T3 and an undetectable thyroglobulin level. Endogenous production of thyroid hormone involves the synthesis of both T4 and T3, as well as the byproduct thyroglobulin. If the patient were hyperthyroid from an endogenous overactive gland (e.g., Graves disease) or destructive thyroiditis, thyroglobulin would be elevated. An undetectable thyroglobulin level points exclusively to an exogenous source, as the suppressed TSH turns off endogenous thyroid gland activity (and thus thyroglobulin production). The specific pattern of low T4 and high T3 indicates that the patient is ingesting pure triiodothyronine (T3, such as liothyronine or certain weight-loss supplements), rather than levothyroxine (T4), which would elevate free T4.

Adım Adım Çözüm

Interpret the thyroid function test pattern.

The patient has thyrotoxicosis with a suppressed TSH, elevated T3, but a paradoxically low free T4.

Establishing the specific pattern of circulating hormones is the first step in differentiating the causes of thyrotoxicosis.

Analyze the thyroglobulin level in the context of thyrotoxicosis.

The thyroglobulin level is undetectable (< 0.5 ng/mL).

Thyroglobulin is a byproduct of endogenous thyroid hormone synthesis. An undetectable level in a thyrotoxic patient confirms an exogenous hormone source, ruling out endogenous overproduction or destructive thyroiditis.

Synthesize the hormone pattern to identify the specific exogenous agent.

The combination of low T4 and high T3 points to pure exogenous T3 ingestion.

Exogenous T3 suppresses TSH, which in turn halts the endogenous thyroid gland's production of T4, explaining the low free T4 level.

Anahtar Kavram

Factitious thyrotoxicosis from T3 ingestion presents with suppressed TSH, elevated T3, paradoxically low T4, and an undetectable thyroglobulin level.

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

Sık Yapılan Hatalar

Konular

Bu konudan daha fazla soru