Determination of TSH levels may be inadequate for assessing thyroid function

What is it about?

Usually, assessment of thyroid function and of adequate substitution therapy with levothyroxine rely on determining thyrotropin (TSH) levels. However, this may result in oversimplification. This trial observed the relation between TSH, FT4, FT3 and sum activity of peripheral deiodinases (SPINA-GD) in a large cohort of 1994 subjects. Results that were confirmed by computer simulations with SimThyr question the loglinear relationship between FT4 and TSH. Thyroid homeostasis may be more complex.

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Why is it important?

The currently predominant TSH-only strategy for assessment of supply with thyroid hormones may be inadequate. This trial suggests T3 production to be a relevant component of pituitary-thyroid feedback control that deserves more attention.