Biochemical Diagnosis of Thyroid Dysfunctions

Thyroid dysfunctions are among the most common endocrine disorders and accurate biochemical testing is integral to assess thyroid patients. Notably, true hyperthyroidism and hypothyroidism in the setting of a normal thyroid-stimulating hormone level are highly unlikely, making the assessment of free thyroxine (FT4) inappropriate in most new cases. However, FT4 measurement is pivotal in both the diagnosis and management of relevant central dysfunctions (central hypothyroidism and central hyperthyroidism) as well as for monitoring therapy in hyperthyroid patients treated with antithyroid drugs or radioiodine. Serum free triiodothyronine (FT3) measurement, however, rarely adds clinically relevant information with the exception of patients with suppressed TSH and normal FT4 levels, in order to confirm or exclude the so-called T3-toxicosis, generally observed in patients with autonomously functioning thyroid nodules and potentially associate with an increased risk of atrial fibrillation. However, technical and interpretative challenges still remain making bidirectional communication between the laboratory and clinical specialists pivotal to properly select reliable methods, establish clinically appropriate reference intervals, investigate discordant results, and monitor the analytical and clinical performances of different methods over time.