Audit of hyperthyroxinaemia and thyrotoxicosis using a sensitive TSH assay

Area of Science:

• Endocrinology
• Clinical Biochemistry
• Thyroidology

Background:

• Hyperthyroxinemia, or elevated thyroxine levels, can be caused by various factors.
• Accurate diagnosis of thyrotoxicosis requires careful biochemical assessment.
• The role of sensitive thyroid-stimulating hormone (TSH) assays in differentiating causes of hyperthyroxinemia is critical.

Purpose of the Study:

• To investigate the causes of hyperthyroxinemia in a general hospital setting.
• To define the biochemical profile of thyrotoxicosis using a sensitive TSH assay.
• To evaluate the diagnostic utility of TSH in conjunction with thyroid hormone levels.

Main Methods:

• Prospective analysis of 8,382 patient samples over one year.
• Measurement of Total T4 (TT4) and TSH in all samples.
• Selective measurement of TT3, free T4, free T3, and thyroxine-binding globulin (TBG) in specific cases.

Main Results:

• Elevated TT4 was observed in 2.6% of patients; 74% were due to thyrotoxicosis, 19% to elevated TBG, and 7% to non-thyroidal illness.
• Thyrotoxicosis (TSH < 0.15 mU/L) occurred in 2.7% of patients, with high TT4 in 71% of these cases.
• A significant number of patients (4%) exhibited suppressed TSH with normal thyroid hormone levels, and elevated TT4 without thyrotoxicosis was frequent.

Conclusions:

• Routine TSH measurement is essential for distinguishing thyrotoxicosis from other causes of elevated TT4.
• Normal TSH levels reliably exclude thyrotoxicosis.
• Suppressed TSH values alone are insufficient for diagnosing thyrotoxicosis or monitoring treatment.