In which case might a very low plasma TSH result not correlate with thyroid status?

A very low plasma TSH result might not correlate with thyroid status in cases following high-dose corticosteroid treatment. Corticosteroids can influence the hypothalamic-pituitary-thyroid (HPT) axis by decreasing the secretion of Thyroid Stimulating Hormone (TSH) and Thyrotropin-releasing hormone (TRH). This suppression occurs regardless of the actual thyroid hormone levels in the body.

In normal physiology, low TSH usually indicates a condition such as hyperthyroidism, where elevated levels of thyroid hormones would suppress TSH production. However, when a patient is on high-dose corticosteroids, the reduced TSH levels may not reflect an underlying hyperthyroid state, as the hormonal feedback loop is disrupted due to the corticosteroid treatment.

In contrast, euthyroid sick syndrome refers to a condition where patients demonstrate abnormal thyroid function tests due to non-thyroid illness rather than primary thyroid dysfunction. Here, TSH might remain low, but it's reflective of a temporary state rather than a true thyroid disorder.

Congenital hypothyroidism implies a primary deficiency of thyroid hormone from birth which typically correlates with elevated TSH levels due to the lack of feedback inhibition.

Elevated TBG (Thyroxine