National recommendations of the Croatian society of medical biochemistry and laboratory medicine: Thyroid function tests from the laboratory point of view

Thyroid function tests (TFTs) - thyroid stimulating hormone (TSH), total triiodothyronine (T3), total thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), thyroid peroxidase antibodies (anti-TPO), thyroglobulin antibodies (anti-Tg), TSH receptors antibodies (anti-TSHR), and thyroglobulin (Tg) - are used to diagnose thyroid disorders and are crucial biomarkers for monitoring and managing thyroid cancer. The 2022 national survey results revealed that thyroid function testing is not standardized among Croatian medical-biochemistry laboratories. Laboratories follow individual protocols at each testing stage, from patient preparation to result reporting. To address this, the Working group for laboratory endocrinology of the Croatian society of medical biochemistry and laboratory medicine has developed recommendations based on the latest national and international guidelines, research and the authors' expert opinion. The document aims to standardize all phases of thyroid function testing, with 7 preanalytical, 12 analytical, and 8 postanalytical recommendations, each supported by expert explanations. While primarily directed at Croatian laboratory professionals, this document is also relevant to other healthcare professionals managing thyroid-related health issues.