The American Thyroid Association's 2024 update to the management of thyroid dysfunction (Bauer/Cooper) restated something simple that labs have been doing piecemeal for years: the reflex pathway is sequential, not parallel. Order TSH first. Only when TSH is abnormal do you reflex to FT4. Only if the pattern looks hyperthyroid do you add FT3.
That ordering matters for two reasons. First, around 95% of thyroid testing requests are TSH-only screens in primary care — running the full panel reflexively is a waste of reagent and reporting bandwidth. Second, sequential testing produces a clearer narrative for the ordering clinician: TSH suppressed + FT4 elevated + FT3 elevated = primary hyperthyroidism, escalate to imaging (US or RAI uptake) per ATA 2015 nodule guidelines.
The reflex chain in plain terms
Step 1. TSH. Reference range varies slightly by population; ATA notes 0.4-4.0 mIU/L is acceptable for most adults, with pregnancy-specific cut-offs at 0.1-2.5 first trimester.
Step 2. If TSH is abnormal (suppressed below 0.4 or elevated above 4.0), reflex FT4. FT4 confirms whether the abnormality is real and whether it is the hyperthyroid (FT4 high) or hypothyroid (FT4 low) direction.
Step 3. Hyperthyroid pattern adds FT3 to distinguish T3-toxicosis variants and grade severity.
Step 4. Suspicion of autoimmune disease (clinical signs, family history, suppressed TSH with normal FT4) adds Anti-TPO and Anti-Tg. Hashimoto's and Graves' typically have Anti-TPO > 100 IU/mL.
Step 5. Nodule on exam or imaging triggers calcitonin (medullary carcinoma surveillance, NCCN Thyroid Carcinoma 2024) plus ultrasound + FNA per ATA 2015 nodule criteria.
What it means for your lab
Configure your LIS reflex rules to match ATA 2024 ordering. Bundle TSH + FT4 + FT3 + Anti-TPO + Anti-Tg in your Thyroid panel as the menu option, but enable the lab tech to release only the parts the clinical context warrants. Reagent cost-savings on a busy general lab are 20-40% versus reflexive full-panel ordering.
Calcitonin is separate. Run it only on patients with a nodule, family history of MEN2, or unexplained diarrhea + flushing. Never reflex it from a routine thyroid panel.