How can I differentiate between Hashimoto's Thyroiditis and other causes of hypothyroidism in a patient presenting with fatigue and weight gain?

To differentiate between Hashimoto's thyroiditis and other causes of hypothyroidism in a patient presenting with fatigue and weight gain, the primary diagnostic approach involves specific blood tests.

Initially, if there is a clinical suspicion of thyroid disease, tests for thyroid dysfunction should be considered ¹. For adults, this typically involves measuring thyroid-stimulating hormone (TSH) alone, and if the TSH is above the reference range, free thyroxine (FT4) should also be measured in the same sample ¹.

To specifically identify Hashimoto's thyroiditis as the cause of hypothyroidism, you should consider measuring thyroid peroxidase antibodies (TPOAbs) for adults with TSH levels above the reference range ¹. The presence of TPOAbs indicates an autoimmune cause, such as Hashimoto's thyroiditis ¹. TPOAbs testing should not be repeated ¹.

If TPOAbs are negative, other causes of hypothyroidism would be considered. Atypical or difficult-to-interpret thyroid function tests (TFTs) may indicate other conditions such as a TSH-secreting pituitary adenoma, end-organ resistance to thyroid hormones, or other thyroid hormone metabolism disorders, which would require specialist assessment ².

It is important to note that thyroid function tests should not be performed during an acute illness unless the illness is suspected to be due to thyroid dysfunction, as acute illness can affect test results ¹. Additionally, patients should be asked about their biotin intake, as high consumption from dietary supplements can lead to falsely high or low test results ¹.