Specimen Collection

SPECIMEN:

Random urine (5 mL) or 24 hour collect.

No acid.

Requests from endocrinologists, O&G specialists and general physicians will be approved

This test may be vetted by a pathologist.

The clinical information for the test must be clearly written on the request form. If clinical information is not provided, or does not provide sufficient justification for the test, the test may be declined.

Declined tests :

If a test is declined, the specimen will be held for a reasonable period (usually 3 weeks but dependant on the stability of the sample). Medical practitioners seeking approval for a declined test should email the on-call Chemical Pathologist ( chemicalpathologist@adhb.govt.nz ) , giving the patient's name and NHI number and the clinical justification for the test. If unable to email, call the on-call Chemical Pathologist via Lablink (09-3078995) and identify yourself as a doctor.

Test vetting policy

Reference Intervals

Units: ug/L

Reference interval :

Adult : 25 - 650

Children (less than 14y): 40 - 750

The reference range quoted is based on the 2.5th to 97.5th centile of an iodine-replete population (ref.1)

Values > 300 ug/l may impair interpretation of thyroid scintiscan and efficacy of radioiodine therapy.

Uncertainty of Measurement: 12%

Turnaround Time: Within 1 week

Performed Weekly.

Assay Method

Principle : Inductively coupled plasma mass spectrometry (ICP-MS)

Instrument : PlasmaQuant MS Elite

Diagnostic Use and Interpretation

Measurement of urinary iodide in individual patients can be useful in::

(1) assessment of iodine status at the time of therapeutic radioiodine administration or iodinated contrast use e.g. Lipiodol

(2) as part of the investigation of some cases of mild hyperthyroidism

(3) assessment of iodine toxicity

Iodine deficiency

The most sensitive index of iodine deficiency is a rise in TSH (however the increased TSH level can still be within the reference interval), and the only reliable way to diagnose iodine deficiency in an individual is to demonstrate a raised TSH level which decreases following iodine supplementation.

Iodine deficiency has no known effects other than on thyroid hormones. There is no good evidence for the existence of a "subclinical" iodine deficiency state.

In New Zealand, iodine deficiency with resulting goitre and/or hypothyroidism is now uncommon, owing to the supplementation of salt with iodine.

The urine iodide test is not appropriate to diagnose iodine deficiency in individuals . Urine iodide levels are variable from day to day in a given patient, and have a low predictive value for iodine deficiency. Even an accurate 24-hour collection cannot be reliably used to assess iodine status in an individual . To determine iodine status in an individual by using urinary iodide, several urine samples collected over a prolonged period of time (covering seasonal variations e.g. 10 repeat random urine samples) are required, which is impractical in routine clinical practice. Urine iodide reflects only recent iodine intake, whereas the thyroid gland can store large amounts of iodine. Using it to test for iodine deficiency is of no benefit to patients, and may give misleading results leading to harmful interventions e.g. excessive iodine supplementation can cause hyperthyroidism and may increase the risk of autoimmune thyroid disease.

Urine iodide levels have been widely used in epidemiology to assess the iodine status of populations. Some laboratories have historically quoted urine iodide reference ranges which are based on population medians. These ranges are meant for population studies, and should not be applied to individuals. The majority of individuals with urine iodide levels which are "low" in relation to guidelines for population medians are not iodine deficient.

References

1. Soldin OP, Soldin SJ, Pezzullo JC. Urinary iodine percentile ranges in the United States. Clin Chim Acta. 2003;328:185-190

2. Zimmermann MB, Jooste PL, Pandav CS. Iodine-deficiency disorders. Lancet. 2008

3. Zimmermann MB. Methods to assess iron and iodine status. Br J Nutr. 2008;99 Suppl 3:S2-9

4. Rasmussen LB, Ovesen L, Christiansen E. Day-to-day and within-day variation in urinary iodine excretion. Eur J Clin Nutr. 1999;53:401-407

5. Wainwright P and Cook P. The assessment of iodine status - populations, individuals and limitations. Ann Clin Biochem. 2019;56(1):7-14

6. Sohn SY et al. Association between excessive urine iodine excretion and failure of radioactive iodine thyroid ablation in patients with papillary thyroid cancer. Thyroid 2013; 26(6): 741-747

7. Reinherdt MJ et al. Influence of urinary iodine excretion on thyroid Technetium-99m pertechnetate uptake with and without TSH suppression: what happens when iodine supply increases? Eur J Nucl Med 1998; 25: 1475-1481

8. Yaming Li. The influence of urinary iodine excretion on thyroid technetium-99m pertechnetate uptake with Graves hyperthyroidism patients. J Nuc Med May 2019; 60 (supp 1) poster 1097

Contact Information

Emails to chemicalpathologist@adhb.govt.nz will receive priority attention from the on-call chemical pathologist.

If the query concerns a specific patient please include the NHI number in your email.

If email is not a suitable option, please contact the on-call chemical pathologist via Lablink (Auckland City Hospital ext. 22000 or 09-3078995).

Individual chemical pathologists may be contacted but will not be available at all times.

After-hours : contact Lablink (Auckland City Hospital ext. 22000 or 09-3078995) or hospital operator for on duty staff after hours.

Dr Samarina Musaad (Clinical Lead) : SamarinaM@adhb.govt.nz ext. 22402

Dr Cam Kyle: CampbellK@adhb.govt.nz ext 22052

Dr Weldon Chiu: WeldonC@adhb.govt.nz ext. 23427

Dr Campbell Heron: CHeron@adhb.govt.nz ext. 23427

Last updated at 09:25:30 22/01/2024