Blood Spot Thyroids + Dried Urine ICPMS Iodine
This panel combines dried blood spot immunoassay of TSH, free T4, free T3, thyroid peroxidase and thyroglobulin antibodies with dried urine inductively coupled plasma mass spectrometry iodine measurement. It enables comprehensive thyroid function, autoimmune, and iodine status assessment via home finger-prick and urine collection, suited to remote care, needle-averse patients, and integrative…
7
Biomarkers
Dried blood spot
Sample
5–7 business days
Turnaround
No
Fasting
What is the Blood Spot Thyroid + Urine Iodine test?
This dual-matrix panel pairs finger-prick dried blood spot measurement of thyroid hormones and autoantibodies with dried urine inductively coupled plasma mass spectrometry for iodine, offering practitioners a minimally invasive, home-collection option for comprehensive thyroid and iodine status assessment. Blood spot technology extends access to patients in remote settings, those averse to venepuncture, or practitioners working outside hospital-based phlebotomy networks, while urine ICP-MS iodine provides the most analytically robust measure of recent intake free from colorimetric assay interference. The panel is ordered to investigate symptoms of thyroid dysfunction — fatigue, weight change, cold or heat intolerance, mood disturbance — when traditional serum testing is impractical, to screen for autoimmune thyroid disease, to monitor replacement therapy in a non-laboratory setting, and to evaluate iodine sufficiency where deficiency or excess supplementation is suspected. Because results are matrix- and platform-specific, practitioners should confirm reference intervals with ZRT Laboratory and not assume interchangeability with serum values.
What does the Blood Spot Thyroid + Urine Iodine test measure?
Thyroid-stimulating hormone is synthesised in anterior pituitary thyrotrophs in response to hypothalamic thyrotropin-releasing hormone and exerts negative feedback control by circulating thyroid hormones. It drives thyroid follicular cell uptake of iodide, synthesis of thyroglobulin, and secretion of thyroxine and triiodothyronine. Dried blood spot TSH correlates well with serum TSH but requires platform-specific validation.
TSH is the single most sensitive screening marker of primary thyroid dysfunction. An elevated value signals primary hypothyroidism; a suppressed value suggests hyperthyroidism or excessive thyroid hormone replacement. Interpretation is strengthened by simultaneous free T4 and free T3 measurement.
Free thyroxine is the unbound, biologically available fraction of the prohormone T4 secreted by the thyroid gland. Most T4 circulates bound to thyroxine-binding globulin, transthyretin, and albumin; only the free fraction enters cells for peripheral conversion to active T3. Dried blood spot free T4 measures the same physiological entity as serum free T4 but uses immunoassay adapted to the matrix.
Free T4 distinguishes primary from central hypothyroidism (low in both, but TSH is elevated in primary and low or inappropriately normal in central) and helps assess the adequacy of levothyroxine replacement. An elevated free T4 with suppressed TSH indicates thyrotoxicosis.
Free triiodothyronine is the active thyroid hormone, generated predominantly by peripheral deiodination of T4 in liver, kidney, and other tissues via type 1 and type 2 deiodinases. It binds nuclear thyroid receptors to regulate metabolism, and its free fraction reflects tissue-available hormone. Dried blood spot fT3 correlates with serum fT3 when platform-validated.
Free T3 assessment is valuable when T4 is normal but symptoms persist, when evaluating conversion efficiency, and in the diagnosis of T3-predominant hyperthyroidism. Low fT3 with normal fT4 may reflect non-thyroidal illness, selenium or other micronutrient deficiency, or polymorphisms in deiodinase genes.
Thyroid peroxidase antibodies are immunoglobulins directed against the enzyme thyroid peroxidase, which catalyses iodination of tyrosine residues on thyroglobulin. TPOAb is the hallmark of autoimmune thyroid disease, particularly Hashimoto thyroiditis, and reflects T-cell-mediated autoimmune attack on thyroid follicles.
Positive TPOAb confirms autoimmune aetiology in hypothyroidism, predicts progression of subclinical hypothyroidism to overt disease, and is associated with higher miscarriage risk in pregnancy. Titre does not correlate tightly with disease activity but persistent positivity confirms autoimmune pathophysiology.
Thyroglobulin antibodies are directed against thyroglobulin, the iodinated glycoprotein scaffold for thyroid hormone synthesis. They are present in a smaller proportion of autoimmune thyroid disease cases than TPOAb, often coexist with TPOAb, and can interfere with serum thyroglobulin measurements used in thyroid cancer surveillance.
Positive TgAb identifies autoimmune thyroid disease when TPOAb is negative, and its presence must be noted when interpreting thyroglobulin results in post-thyroidectomy monitoring. In functional practice, dual positivity (TPOAb and TgAb) signals a robust autoimmune process.
Iodine is an essential trace element and the rate-limiting substrate for thyroid hormone synthesis. Dietary iodine is absorbed in the small bowel, concentrated by the thyroid via the sodium-iodide symporter, and excess is excreted renally. Urinary iodine reflects recent intake; inductively coupled plasma mass spectrometry (ICP-MS) provides high sensitivity and specificity, free from the matrix effects seen in some colorimetric assays.
Urinary iodine is the preferred biomarker of population iodine status and the only practical measure of individual intake, though a single spot sample has limited precision due to biological and dietary variation. Low values support iodine deficiency as a contributor to hypothyroidism or goitre; high values may reflect excess supplementation. Interpretation is improved by concurrent thyroid function tests and by repeat sampling.
The free T3 to free T4 ratio reflects the efficiency of peripheral conversion of T4 to T3 by deiodinase enzymes. A low ratio may indicate impaired conversion due to illness, nutritional deficiency (selenium, zinc, iron), inflammation, or medication (beta-blockers, corticosteroids), while a high ratio can occur in hyperthyroidism or preferential T3 secretion.
The fT3:fT4 ratio is used in functional and integrative practice to identify conversion insufficiency when fT4 is normal but fT3 is disproportionately low. It guides decisions about combination T4/T3 therapy, though mainstream endocrinology does not universally endorse this metric.
Why is the Blood Spot Thyroid + Urine Iodine test ordered?
- Investigation of suspected hypothyroidism or hyperthyroidism in patients who prefer home collection or lack access to venepuncture services
- Screening for autoimmune thyroid disease (Hashimoto or Graves) in patients with thyroid dysfunction, family history, or other autoimmune conditions
- Monitoring of thyroid hormone replacement therapy in patients on levothyroxine, liothyronine, or desiccated thyroid, particularly when assessing T4-to-T3 conversion adequacy
- Evaluation of iodine status in regions of known deficiency, in patients on restricted diets (vegan, low-sodium, avoidance of iodised salt), or when excessive supplementation is suspected
- Assessment of persistently symptomatic patients on levothyroxine monotherapy whose serum TSH is normalised but free T3 may be suboptimal
- Integrative or functional medicine contexts where home sample collection, dried specimen stability, and combined thyroid-iodine evaluation are prioritised
- Paediatric or needle-phobic populations where finger-prick collection reduces distress and improves compliance
- Serial monitoring in telehealth or remote-care models where laboratory attendance is a barrier
Sample collection and turnaround
Sample type
Dried blood spot, Urine (dried)
Fasting required
No
Collection method
Finger prick
Turnaround
5–7 business days
Collection notes
Patient self-collects a finger-prick blood spot on a filter paper card supplied in the ZRT home kit, and applies a few drops of urine to a separate dried urine collection card. Both specimens are air-dried at room temperature per kit instructions, then mailed to ZRT Laboratory. Practitioners should instruct patients to collect the sample before taking thyroid medication (or at a consistent time relative to dosing) and to follow their usual diet unless iodine-loading assessment is specifically desired. Biotin supplementation should be discontinued at least 72 hours before collection if the assay is biotin-sensitive.
Specimen requirements
Complete saturation of blood spot circles (typically four to six spots) and urine application to the designated area on the dried urine card. Specimens must be fully air-dried (minimum 2–4 hours, away from direct heat) before sealing in the foil pouch. Incomplete drying, excessive humidity, or contamination can compromise assay accuracy.
What can affect Blood Spot Thyroid + Urine Iodine results?
Urinary iodine reflects intake over the preceding hours to days, so recent high-dose supplementation or seaweed consumption will transiently elevate the result and may not represent habitual status; advise patients to follow their usual diet unless instructed otherwise.
High-dose biotin can interfere with streptavidin-biotin immunoassays used for thyroid hormones, causing falsely high or low TSH, fT4, and fT3. Discontinuation for at least 72 hours before collection is recommended if the assay platform is biotin-sensitive; confirm with ZRT.
Levothyroxine taken shortly before blood collection will transiently raise fT4; liothyronine raises fT3 within hours. Practitioners should standardise collection timing relative to medication (typically before the morning dose or at a consistent interval) for serial comparability.
Systemic illness, fasting, or severe stress suppress peripheral conversion of T4 to T3, lowering fT3 and the fT3:fT4 ratio, and can alter TSH. Results during acute illness may not reflect true thyroid status and should be repeated after recovery.
Dried blood spot hormone concentrations are influenced by haematocrit and the completeness of spot saturation; overfilled, underfilled, or incompletely dried cards can yield inaccurate results. Clear collection instructions and quality-control checks at the laboratory mitigate this.
Urinary iodine concentration varies inversely with urine volume, so well-hydrated (dilute) samples will have lower iodine per litre and dehydrated samples higher. Reporting iodine per gram creatinine partially corrects for this, though creatinine excretion itself varies by muscle mass and diet.
What do Blood Spot Thyroid + Urine Iodine results mean?
Elevated
TSH elevated with low or low-normal fT4 indicates primary hypothyroidism; positive TPOAb or TgAb confirms autoimmune aetiology. Elevated fT4 and fT3 with suppressed TSH indicates hyperthyroidism or over-replacement. Urinary iodine above 300 µg/L suggests excess intake and, if persistent, may contribute to autoimmune thyroid disease or iodine-induced thyrotoxicosis in susceptible individuals.
Normal
TSH, fT4, and fT3 within reference limits, with negative thyroid antibodies and urinary iodine 100–299 µg/L, indicate euthyroid status and adequate iodine intake. However, a normal TSH does not exclude tissue hypothyroidism or conversion insufficiency if symptoms persist; some practitioners assess the fT3:fT4 ratio and consider free hormone levels in the context of individual symptom response.
Reduced
TSH suppressed with elevated fT4 and fT3 indicates thyrotoxicosis; suppressed TSH with low or low-normal free hormones suggests central hypothyroidism or sick euthyroid syndrome and warrants endocrine referral. Low fT3 with normal fT4 and TSH may reflect impaired peripheral conversion; calculate the fT3:fT4 ratio and investigate selenium, zinc, or ferritin deficiency, systemic illness, or medication effects. Urinary iodine below 100 µg/L supports iodine insufficiency as a contributor to thyroid dysfunction, particularly in regions without universal salt iodisation.
Note: Dried blood spot reference intervals are method- and platform-specific and are not interchangeable with serum values from hospital laboratories. Serial results should be compared within the same laboratory and matrix. Urinary iodine from a single spot sample has wide day-to-day variability and is best interpreted as a population marker or with repeat samples in individuals. The fT3:fT4 ratio is used more commonly in integrative practice than in mainstream endocrinology; evidence for conversion-insufficiency thresholds is limited. Biotin interference, medication timing, acute illness, and sample collection quality all affect results.
Frequently asked questions
Dried blood spot TSH, free T4, and free T3 correlate well with serum when platform-validated, but reference intervals are method-specific and not interchangeable. Serial results should be compared within the same laboratory and specimen type, and initial diagnosis or precise interval matching may require serum.
How to access the Blood Spot Thyroid + Urine Iodine test
The Blood Spot Thyroid + Urine Iodine reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Jonklaas J, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. (2014)
- Pearce EN, et al. AACE/ACE Position statement on thyroid testing in asymptomatic, non-pregnant adults. Endocr Pract. (2020)
- WHO, UNICEF, ICCIDD. Assessment of iodine deficiency disorders and monitoring their elimination: a guide for programme managers. Third edition. (2007)
- Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol. (2015)
- Stagnaro-Green A, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. (2011)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
Related tests
thyroid endocrine
TSH
The TSH test measures Thyroid Stimulating Hormone produced by the anterior pituitary gland. It is the recommended first-line screening test for thyroid dysfunction, with elevated TSH indicating primary hypothyroidism and suppressed TSH indicating hyperthyroidism. TSH alone does not assess T4-to-T3 conversion or exclude central hypothyroidism.
thyroid endocrine
Free T4
The Free T4 test measures free thyroxine, the unbound and metabolically available fraction of T4, in serum. Interpreted alongside TSH, it confirms and localises thyroid dysfunction — diagnosing primary and central hypothyroidism, grading hyperthyroidism, and monitoring levothyroxine therapy. Free T4 has replaced Total T4 in routine practice because it is unaffected by binding-protein changes.
thyroid endocrine
Free T3
The Free T3 test measures free triiodothyronine, the unbound and biologically active fraction of T3, in serum. It is primarily indicated for assessing the severity of hyperthyroidism, detecting T3 toxicosis, and characterising non-thyroidal illness syndrome. TSH and Free T4 remain the first-line markers for thyroid screening and hypothyroidism.