Thyroid disorders are common, affecting more than 10% of people in the US, and laboratory tests are integral in the management of these conditions. The repertoire of thyroid tests includes blood tests for thyroid-stimulating hormone (TSH), free thyroxine, free triiodothyronine, thyroglobulin (Tg), thyroglobulin antibodies (Tg-Ab), thyroid peroxidase antibodies (TPO-Ab), TSH receptor antibodies (TRAb), and calcitonin. TSH and free thyroid hormone tests are frequently used to assess the functional status of the thyroid. TPO-Ab and TRAb tests are used to diagnose Hashimoto's thyroiditis and Graves' disease, respectively. Tg and calcitonin are important tumor markers used in the management of differentiated thyroid carcinoma and medullary thyroid carcinoma (MTC), respectively. Procalcitonin may replace calcitonin as a biomarker for MTC. Apart from understanding normal thyroid physiology, it is important to be familiar with the possible pitfalls and caveats in the use of these tests so that they can be interpreted properly and accurately. When results are discordant, clinicians and laboratorians should be mindful of possible assay interferences and/or the effects of concurrent medications. In addition, thyroid function may appear abnormal in the absence of actual thyroid dysfunction during pregnancy and in critical illness. Hence, it is important to consider the clinical context when interpreting results. This review aims to describe the above-mentioned blood tests used in the diagnosis and management of thyroid disorders, as well as the pitfalls in their interpretation. With due knowledge and care, clinicians and laboratorians will be able to fully appreciate the clinical utility of these important laboratory tests.
A previously well 32-year-old Chinese male presented with acute bilateral upper and lower limb paralysis upon waking, ten days after the onset of COVID-19 infection. Examination revealed areflexia over all four limbs, associated with reduced muscle strength, but no sensory or cranial nerve deficit. Initial concern was Guillain-Barre syndrome given the acute flaccid paralysis following COVID-19 infection. However, investigations revealed severe hypokalaemia (1.7 mmol/L) and primary hyperthyroidism. He was treated for thyrotoxic periodic paralysis (TPP) with β-blockers, antithyroid medications, and intravenous potassium chloride (KCl). Despite frequent monitoring of potassium, rebound hyperkalaemia occurred with prompt resolution of paralysis.
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