Thyroid tests in kidney failure: diagnostic stewardship before levothyroxine therapy

Advanced chronic kidney disease and kidney failure frequently produce thyroid-test patterns that resemble primary hypothyroidism. The clinical challenge is not to dismiss abnormal results, but to distinguish persistent, treatment-requiring thyroid failure from uraemic, non-thyroidal, volume-related, medication-related, and analytical mimics before committing a vulnerable patient to long-term levothyroxine. This structured, evidence-informed narrative review critically appraises epidemiology, dialysis-specific sampling studies, prognostic cohorts, treatment evidence, and major thyroid guidance. Low triiodothyronine is consistently associated with inflammation, protein-energy wasting, and adverse outcomes, but currently functions chiefly as a prognostic and illness-severity signal rather than a replacement target. For clinically stable patients without a red flag, we propose repeat thyroid-stimulating hormone and free-thyroxine testing after optimisation of volume status and systemic illness, at a consistent point in the dialysis cycle, with review of iodine exposure, medicines, absorption, and assay interference. General-population thresholds for subclinical hypothyroidism may inform, but cannot replace, contextual judgement because no chronic-kidney-disease- or dialysis-specific diagnostic or treatment threshold has been prospectively validated. Prompt treatment or endocrinology input remains appropriate for myxoedema coma, central hypothyroidism, pregnancy or pregnancy planning, known permanent hypothyroidism, and a persistent overt primary pattern. The goal is diagnostic precision: timely levothyroxine for patients most likely to benefit and avoidance of premature long-term replacement for reversible mimicry.