dromlakhani Select TSHD screening tests
STEP 1 — Gather Information
Collect history of cranial/spinal radiation dose to the hypothalamic–pituitary axis, tumor or surgery involving the HP region, time since treatment, presence of other anterior pituitary deficits, and any symptoms suggestive of central hypothyroidism (e.g., fatigue, weight gain, cold intolerance).
Action: Determine if the survivor meets criteria for TSHD screening (HP radiation ≥30 Gy or HP tumor/surgery).
STEP 2 — Rule In / Rule Out
If the survivor received ≥30 Gy hypothalamic–pituitary radiation or has a tumor/surgery involving the HP region → proceed to screening; otherwise, routine TSHD screening is not indicated unless symptomatic or other clinical concerns arise.
Decision: Screen for TSHD only in at‑risk survivors.
STEP 3 — Classify or Stratify
For at‑risk survivors, select the same biochemical tests used in the noncancer population: measure free T4 (fT4) and TSH using a reliable assay (e.g., immunoassay or equilibrium dialysis if antiepileptics are used).
Decision: Order fT4 and TSH annually.
STEP 4 — Decide
Obtain fT4 and TSH at least once per year; interpret using the same reference ranges as noncancer peers. A low‑normal or below‑normal fT4 with a normal, low‑normal, or mildly elevated TSH suggests TSHD and warrants endocrinology referral for possible levothyroxine replacement; normal fT4 and TSH make TSHD unlikely.
Action: Annual fT4/TSH testing with appropriate follow‑up based on results.
Clinical Guardrails / Mimics / Pitfalls
- Do not rely on TSH alone; TSH is unreliable for monitoring thyroid hormone replacement in central hypothyroidism.
- Avoid using TSH surge analysis, TRH stimulation, or serum triiodothyronine for diagnosis.
- Use the same assay and reference ranges as for the general population; do not employ special “cancer survivor” ranges.
- Be aware of assay interference (e.g., phenytoin, carbamazepine) that can artifactually lower fT4; confirm low fT4 with equilibrium dialysis if needed.
- Do not omit screening in survivors with HP radiation ≥30 Gy even if asymptomatic, as TSHD may develop years later.
Concrete Clinical Example
A 16‑year‑old survivor of medulloblastoma received 35 Gy craniospinal irradiation at age 5 and is asymptomatic for thyroid dysfunction. According to the skill, order free T4 and TSH annually using the same assay and reference ranges as in noncancer adolescents; results are fT4 1.2 ng/dL (normal low‑normal) and TSH 2.1 µIU/mL (normal), prompting continued annual surveillance.
Source: Hypothalamic Pituitary and Growth Disorders in Survivors of Childhood Cancer: An Endocrine Society Clinical Practice Guideline, Endocrine Society, 2018, DOI:10.1210/jc.2018-01175