Download (ICP-MS) Method for Measurement of Urine Iodine

Development of an Inductively Coupled Plasma-Mass
Spectrometry (ICP-MS) Method for Measurement of Urine Iodine
& Assessment of Iodine Status in Subclinical Hypothyroidism
K Jones1, J Rogers1, A DeLloyd2, DA Rees3, M Ludgate2, C Evans1
1Medical
Biochemistry & Immunology, University Hospital of Wales, Cardiff, UK. Email: [email protected]
Research Group, Institute for Molecular & Experimental Medicine, School of Medicine, Cardiff University, UK
3Cardiovascular & Metabolism Research Group, Institute for Molecular & Experimental Medicine, Cardiff University, UK
2Thyroid
Introduction
A
B
Linearity
127I
ISTD:125Te
y=0.0049x+0.0085
• Standard curves were linear
with a correlation coefficient
consistently >0.999 (figure 2)
• Dilution of urine iodine was
linear over the range 1–2527 µg/L
R=1.0000
Ratio
• Iodine (I) is a component of thyroid hormones
thyroxine (T4) and triiodothyronine (figure 1)
Method Validation
500
1000
Iodine concentration (ppb)
Figure 2: Representative standard curve
Figure 1: Structure of (A) thyroxine, (B) triiodothyronine
• Iodine deficiency may lead to reduced thyroid
hormone production & consequences including
goitre, hypothyroidism, foetal abnormalities, mental
retardation & susceptibility to iodine-induced
hyperthyroidism1
• A recent study suggests the UK may be iodine
deficient2, indicating the need for further research
• Population iodine status can be assessed by
measurement of urine iodine1, for which ICP-MS is
the gold standard method3
Aims
1. To develop & validate a method for
measurement of urine iodine by ICP-MS
2. To apply this method to a local population
with subclinical hypothyroidism
Methods
ICP-MS
Analysis was carried out on an Agilent 7700x ICPMS with autosampler and integrated sample
introduction system. Helium gas (5mL/min) was
used in the collision cell and iodine was monitored
at m/z 127. Aqueous iodine standards 0-1000µg/L
(0-7800nmol/L) (Inorganic Ventures) were used for
calibration. Standards, samples and quality
control material were diluted 1 in 20 in diluent (1%
TMAH, 100ppb 125Te (Inorganic Ventures), 0.01%
Triton-X). Quality control material consisted of
Seronorm Urine Level 2 (Alere Ltd) and urine pools
(3 levels) prepared in house. The laboratory
enrolled in the CDC EQUIP (Ensuring the Quality
of Urinary Iodine Procedures) EQA scheme.
Accuracy
Sensitivity
• LLOD 2 µg/L (17 nmol/L)
• LLOQ 1 µg/L (8 nmol/L)
Recovery
Iodine concentration (µg/L)
% recovery
200
400
600
800
1000
120
107
107
104
103
Figure 3: Difference plot showed no
method bias compared to CDC target
values in sixteen EQA samples
Table 2: Recovery of iodine in urine
Precision
Carryover
Urine iodine
concentration
(µg/L)
21
Intra-assay
CV (%)
Inter-assay
3.6
3.0
286
788
2.2
1.7
2.1
1.1
No carryover was observed during
analysis of blank samples following
the top standard (1000 µg/L) or urine
samples (>2500 µg/L)
Table 1: Intra- and inter-assay CV
Iodine status in subclinical hypothyroidism
• The population median urine iodine concentration was 94 µg/L (0.73 µmol/L),
consistent with mild iodine deficiency1
• A wide range of iodine excretion was observed: 8–3341 µg/L (0.06–26.33 µmol/L)
• No correlation was observed between urine iodine and TSH (figure 4) or free T4
Figure 4: Scatter plot showing relationship
between urine iodine and TSH (n=203)
Subclinical hypothyroidism study
Spot urines were collected from 203 individuals
(20-71 years (median 54.5 years), 75% female,
90% Caucasian) enrolled in a local study on
subclinical hypothyroidism (selection criteria TSH
5-10mU/L, fT4 within reference range). Ethical
approval was granted for this study. Participants
had repeat thyroid function (TSH & fT4) at the time
of urine collection.
References
Mild deficiency
Moderate deficiency
Severe deficiency
Summary
1World Health
Organization (2007). Assessment of iodine deficiency disorders and
monitoring their elimination. www.who.int/nutrition/publications/en/
2Vanderpump, Lazarus, Smyth et al. (2011). Lancet. 377:2007-12
3Jooste & Strydom (2010). Best Pract Res Clin Endocrinol Metab. 24:77-88
Acknowledgements:
Steven Smith (University Hospital of Wales, UK)
• An ICP-MS method for urine iodine has been successfully developed and validated
• Application of this method revealed mild iodine deficiency in a local study population
• Urine iodine does not correlate with TSH or fT4 in this population