A 36-year-old woman with a past medical history of fibromyalgia presented to her general physician for a regular check-up. The patient reported that her mother was recently diagnosed with hypothyroidism and requested that her scheduled blood work include thyroid function tests. The patient said she has been well; even her symptoms related to fibromyalgia had been under control. Her only complaint was some hair loss, but it had slightly improved since she began using over-the-counter supplements 3 months before her appointment.

On physical examination, the patient appeared to be healthy: her vital signs were normal, there was no evidence of thyroid eye disease, tremor, or tachycardia; and her thyroid gland was not enlarged. Blood work completed the same day revealed normal complete blood count and liver and kidney function tests, but thyroid-stimulating hormone (TSH) was low, measuring 0.02 µIU/mL (reference range, 0.4-4.6 mIU/L). Free thyroxine (T4) was measured at 24 pmol/L (reference range, 10-19 pmol/L), and free triiodothyronine was measured at 7.1 pmol/L (reference range, 3.5-6.5 pmol/L). After review of her previous laboratory tests, it was found that her thyroid function tests, including TSH and free T4 levels, were previously normal on several occasions.

In a follow-up visit, the patient denied recent pregnancy, iodine exposure, neck pain or fever, recent acute illness, and symptoms of thyrotoxicosis. In addition, she denied receiving any new medication, specifically amiodarone or lithium.

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