Triiodothyronine is normally much lower than thyroxine (approximately 100 fold). In hypothyroidism the ratio is increased since thyroxine is reduced more than triiodothyronine.
triiodothyronine-to-thyroxine ratio =
= (triiodothyronine in ng/dL) / (thyroxine in µg/dL)
where:
• It is necessary to keep track of units (1 µg = 1000 pg).
Interpretation:
• in hypothyroidism, the ratio is high
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