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Thyroid function which is particularly useful in the diagnosis of T₃ thyrotoxicosis, in which T₃ is increased and T₄ is within normal limits. T₃ toxicosis is occasionally found in Graves’ disease. It occurs with a single toxic nodule, multinodular thyrotoxicosis, and following treatment with T₃ (Cytomel®).² It is increased in and occasionally helpful for confirmation of diagnosis of conventional hyperthyroidism, in which commonly both T₃ and T₄ levels are increased. T₃ is needed in patients with clinical evidence for hyperthyroidism, in whom the usual thyroid profile is normal or borderline.

Reported to be normal to slightly increased with familial dysalbuminemic hyperthyroxinemia.

Recommended for patients with supraventricular tachycardia, for patients with fatigue and weight loss not otherwise explained, or for those with proximal myopathy and in whom T₄ levels are not elevated.³