Differential Diagnosis
- Single most likely diagnosis
- Exogenous thyrotoxicosis: This is the most likely diagnosis due to the patient's low thyroid-stimulating hormone (TSH) concentration, elevated free thyroxine (FT4) level, and very low radioactive iodine uptake on the thyroid scan. The patient's occupation as a certified nursing assistant increases the likelihood of access to and ingestion of exogenous thyroid hormone, which could explain the thyrotoxicosis symptoms without evidence of thyroid gland abnormalities.
- Other Likely diagnoses
- Graves disease: Although less likely given the very low iodine uptake, Graves disease is a common cause of hyperthyroidism. However, the absence of a palpable goiter, exophthalmos, and the low iodine uptake make this diagnosis less probable.
- Painless thyroiditis: This condition can cause thyrotoxicosis due to the release of preformed thyroid hormones from the inflamed gland. However, the normal ultrasonography and very low iodine uptake are not typical for painless thyroiditis, which usually shows some inflammation on imaging and has a higher iodine uptake initially before it decreases.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Thyroid storm: Although not a primary diagnosis but a complication, thyroid storm is a life-threatening condition that requires immediate recognition and treatment. The patient's symptoms of palpitations, anxiety, and tremor could be indicative of thyrotoxicosis, and any worsening could potentially lead to a thyroid storm.
- Rare diagnoses
- Chronic lymphocytic (Hashimoto) thyroiditis with transient thyrotoxicosis: While Hashimoto's can cause hypothyroidism, a transient phase of thyrotoxicosis can occur due to the release of thyroid hormones from the damaged gland. However, the very low iodine uptake and lack of thyroid gland abnormalities on ultrasonography make this less likely.
- Goiter: The question stem mentions that the thyroid gland is not palpable, and ultrasonography shows no abnormalities, making goiter an unlikely diagnosis.
- Thyroglobulin-related issues: The thyroglobulin level is within the normal range, and there's no other indication of thyroglobulin-related pathology, making this an unlikely diagnosis.