From the Guidelines
To confirm a diagnosis of thyrotoxicosis, the most appropriate laboratory tests include thyroid function tests, specifically a low thyroid stimulating hormone (TSH) level and elevated free thyroxine (T4) and/or triiodothyronine (T3) levels, as recommended by the most recent guidelines 1. These tests should be ordered together as a panel to provide a complete picture of thyroid function. In some cases, TSH may be suppressed while T4 is normal but T3 is elevated (T3 toxicosis). Additional helpful tests include thyroid antibodies such as thyroid stimulating immunoglobulins (TSI) or thyroid peroxidase antibodies (TPO) to determine the underlying cause, particularly if Graves' disease is suspected 1. Some key points to consider when evaluating thyrotoxicosis include:
- TSH and FT4 should be used for case detection in symptomatic patients, with T3 being helpful in highly symptomatic patients with minimal FT4 elevations 1
- A radioactive iodine uptake measurement and scan can distinguish between destructive thyroiditis and hyperthyroidism due to Graves disease or nodular goiter 1
- Thyroid radioactive iodine uptake scan may also be useful to differentiate between causes of thyrotoxicosis, such as Graves' disease versus thyroiditis 1 These tests are essential because thyrotoxicosis represents a state of excess thyroid hormone activity in the body, which can be caused by various conditions, and proper diagnosis guides appropriate treatment. Results should be interpreted in the clinical context of the patient's symptoms, which typically include tachycardia, tremor, weight loss, heat intolerance, and anxiety.
From the Research
Diagnosis of Thyrotoxicosis
To confirm the diagnosis of thyrotoxicosis, several laboratory tests can be used, including:
- Measurement of serum free thyroid hormone concentrations, such as free triiodothyronine (FT3) and free thyroxine (FT4) 2
- Determination of thyroid-stimulating hormone (TSH) levels 3, 4, 5, 6
- Thyroid ultrasound, thyroid scintigraphy, and radioactive iodine uptake (RAIU) tests can also be helpful in the differential diagnosis of thyrotoxicosis and hyperthyroidism 4
- Antithyroid antibody assay can be used to confirm the diagnosis of autoimmune thyroid diseases, such as Graves' disease 3
Laboratory Tests
The following laboratory tests can be used to confirm the diagnosis of thyrotoxicosis:
- Serum FT3 and FT4 levels: elevated levels can indicate thyrotoxicosis 2, 4, 5
- TSH levels: low to undetectable levels can indicate thyrotoxicosis 3, 4, 5, 6
- FT3/FT4 ratio: a ratio of >2.96 can indicate Graves' disease, while a ratio of <2.96 can indicate destructive thyroiditis 4
- Thyroid receptor antibody (TRab) test: positive results can indicate Graves' disease 6
Differential Diagnosis
The following tests can be used to differentiate between different causes of thyrotoxicosis:
- FT3/FT4 ratio: can help differentiate between Graves' disease and subacute thyroiditis 4, 5
- Thyroid scintigraphy and RAIU tests: can help differentiate between Graves' disease and destructive thyroiditis 4
- Antithyroid antibody assay: can help confirm the diagnosis of autoimmune thyroid diseases, such as Graves' disease 3