HCPCS A9531: Iodine I-131 Sodium Iodide, Diagnostic (per microcurie)

HCPCS Level II code A9531 denotes diagnostic Iodine I-131 sodium iodide billed per microcurie (up to 100 microcuries). This radiopharmaceutical code is used for diagnostic nuclear medicine procedures—most commonly thyroid imaging and functional uptake studies—that rely on low-activity radioactive iodine to assess thyroid anatomy and physiology. The code matters nationally because radiopharmaceutical utilization, appropriate billing, and payer coverage policies affect access to diagnostic thyroid evaluation and influence facility billing practices for nuclear medicine services.

Key payers included in this analysis are Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of clinical context for A9531, typical sites of service, and the types of measures and benchmarks commonly reported for radiopharmaceuticals. The publication outlines payer coverage considerations, common billing modifiers used with diagnostic radiopharmaceuticals (listing provided separately), and the policy and reimbursement elements that commonly affect claims processing for low-activity I-131 diagnostic administrations. The report also summarizes what clinicians and billing teams should expect regarding coding conventions, documentation drivers, and national-level implications for utilization and reimbursement trends. Data not available in the input will be noted where applicable.

Billing Code Overview

HCPCS Level II code A9531 describes Iodine I-131 sodium iodide, diagnostic, per microcurie (up to 100 microcuries). This code represents the diagnostic radiopharmaceutical preparation used for nuclear medicine thyroid imaging or functional studies involving radioactive iodine at diagnostic activity levels.

Service type: Diagnostic radiopharmaceutical service involving administration of radioactive iodine for imaging or functional assessment.

Typical site of service: Nuclear medicine clinics, hospital outpatient imaging departments, and freestanding imaging centers where diagnostic radiopharmaceuticals are administered and imaging or uptake measurements are performed.

Clinical Context

A typical patient is an adult referred to nuclear medicine for diagnostic evaluation of thyroid function or residual thyroid tissue after thyroidectomy. The patient presents with a history of hyperthyroidism, differentiated thyroid carcinoma follow-up, or an indeterminate radioactive iodine uptake requiring low‑dose imaging. The clinical workflow: the ordering clinician documents an indication (for example, suspected Graves disease, toxic multinodular goiter, or postoperative surveillance of thyroid cancer) and an ICD-10 diagnosis. The nuclear medicine team performs patient screening for recent iodine exposure, pregnancy, and breastfeeding. On the scheduled day, a measured diagnostic dose of I-131 sodium iodide is prepared and dispensed per microcurie billing; the patient ingests the tracer (typically up to 100 microcuries). Uptake and imaging occur at institution-specific intervals (often 24 hours and/or 48 hours), using a gamma camera or uptake probe. Results are interpreted by a nuclear medicine physician or an authorized radiologist and reported back to the referring provider for clinical management decisions such as medical therapy adjustment, further imaging, or therapeutic planning.

Coding Specifications

Modifier	Description	When to Use
`26`	Professional component	Use when only the professional interpretation/reporting is billed separate from technical services.

Taxonomy Code	Specialty	Notes
`207K00000X`	Nuclear Medicine Physician	Primary specialty responsible for interpretation and reporting of diagnostic `I-131` studies.
`208000000X`	Diagnostic Radiology	Radiologists with nuclear medicine training frequently interpret low‑dose I‑131 imaging.
`363A00000X`	Radiology Technologist	Performs imaging acquisition and handles radiopharmaceutical administration under protocol.
`363L00000X`	Nuclear Medicine Technologist	Prepares and administers the diagnostic `I-131` tracer and performs uptake measurements.
`371K00000X`	Clinical Laboratory Scientist	May be involved in radiopharmaceutical quality control and dose assays.

Related Diagnoses

ICD-10 Code	Description	Clinical Relevance
`E05.00`	Thyrotoxicosis with diffuse goiter, without thyrotoxic crisis or storm	Used when evaluating hyperthyroidism (eg, Graves disease) with diagnostic `I-131` uptake to quantify function.
`E05.10`	Thyrotoxicosis with toxic single thyroid nodule	Used to assess uptake in a suspected toxic adenoma.
`E05.20`	Thyrotoxicosis with toxic multinodular goiter	Used to evaluate nodular hyperfunction and plan management.
`C73`	Malignant neoplasm of thyroid gland	Used for postoperative surveillance to detect residual thyroid tissue or metastatic iodine‑avid disease with diagnostic `I-131`.
`E04.1`	Nontoxic single thyroid nodule	Used when a nonfunctioning nodule requires functional imaging to differentiate cold vs. hot nodules.
`E06.9`	Thyroiditis, unspecified	Used when inflammatory thyroid disease requires assessment of uptake patterns.
`R94.5`	Abnormal results of thyroid function studies	Used when abnormal labs prompt functional imaging with diagnostic `I-131`.
`Z86.73`	Personal history of malignant neoplasm of thyroid	Used during long‑term surveillance where low‑dose `I-131` imaging may be indicated.

Related CPT Codes

CPT Code	Description	Relationship to This Procedure
`78015`	Thyroid uptake; uptake only (quantitative)	Performed when quantitative thyroid uptake measurement is required in conjunction with `I-131` diagnostic tracer administration.
`78070`	Thyroid imaging; planar	Used when planar imaging of the thyroid gland is obtained after ingestion of diagnostic `I-131` to assess anatomy and uptake distribution.
`78800`	Radiopharmaceutical therapy administration (injection)	Related when transitioning from diagnostic to therapeutic radiopharmaceutical administration workflows; documents administration procedures though therapeutic doses differ from diagnostic microcurie dosing.
`76999`	Ultrasound procedure, unlisted — guidance for localization (unlisted)	May be used adjunctively for ultrasound localization prior to or after nuclear imaging when anatomical correlation is needed.
`93000`	Electrocardiogram, routine ECG with interpretation and report	Performed pre-procedure in patients with significant comorbidities when baseline cardiac monitoring is required prior to administration.

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Summary & Overview