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 may increase her access to thyroid hormones, and her symptoms of palpitations, anxiety, and fine resting tremor are consistent with thyrotoxicosis caused by exogenous thyroid hormone ingestion.
- Other Likely diagnoses
- Graves disease: Although less likely due to the very low radioactive iodine uptake, Graves disease could still be considered, especially given the patient's symptoms. However, the absence of a palpable goiter and the low iodine uptake make this less likely.
- Painless thyroiditis: This condition can cause thyrotoxicosis due to the release of pre-formed thyroid hormones from the inflamed gland. However, the normal ultrasonography of the thyroid gland and the very low iodine uptake make this diagnosis less likely.
- Do Not Miss diagnoses
- Thyroid storm: Although not a primary diagnosis, thyroid storm is a life-threatening condition that can occur in untreated or undertreated thyrotoxicosis. It is crucial to recognize and manage promptly.
- Toxic multinodular goiter: While the ultrasonography shows no abnormalities, and the patient does not have a palpable goiter, a toxic multinodular goiter could still be considered, especially if the patient has a history of iodine deficiency or exposure. However, the very low iodine uptake and lack of goiter on examination make this less likely.
- Rare diagnoses
- Struma ovarii: A rare ovarian tumor that can produce thyroid hormones, leading to thyrotoxicosis. This diagnosis would be considered if other causes are ruled out and there is evidence of an ovarian mass.
- Familial dysalbuminemic hyperthyroxinemia: A rare genetic condition that affects the binding of thyroxine to albumin, leading to elevated FT4 levels without true thyrotoxicosis. This diagnosis would require specific testing to confirm.