Endocrine Assay Validation: T4, Cortisol, Progesterone, Relaxin¶
1. Scope¶
Endocrine testing is a cornerstone of veterinary internal medicine, but hormone assays are analytically demanding: analytes are present at low concentrations, are often unstable pre-analytically, and reference ranges are species- and context-dependent. This document summarizes the validation of Migibio's four core endocrine assays.
| Assay | Primary Use | Species |
|---|---|---|
| T4 (Total Thyroxine) | Hypo/hyperthyroidism screening | Canine, Feline |
| TSH | Hypothyroidism confirmation | Canine |
| Cortisol | Cushing's / Addison's (adrenal) | Canine |
| Progesterone | Breeding / ovulation timing | Canine |
| Relaxin | Pregnancy confirmation | Canine, Feline |
2. Clinical Rationale¶
2.1 Thyroid — T4 & TSH¶
- T4 is the primary screening test for thyroid function in dogs and cats.
- TSH is measured in combination with T4 to differentiate primary hypothyroidism (high TSH + low T4) from euthyroid sick syndrome.
- Feline hyperthyroidism is diagnosed by elevated T4; ~10% of early cases have normal T4 but elevated free T4.
2.2 Cortisol — Adrenal Function¶
- Basal cortisol alone is insufficient; dynamic testing (ACTH stimulation / low-dose dexamethasone suppression) is standard.
- FICT provides rapid quantitative cortisol to support same-visit adrenal evaluation.
2.3 Progesterone — Breeding Management¶
- Progesterone is the gold standard for ovulation timing — the single highest-value endocrine test in breeding practice.
- Serial measurement (every 2–3 days) pinpoints the LH surge and optimal mating/AI window.
2.4 Relaxin — Pregnancy¶
- Relaxin is produced by the placenta; detection confirms pregnancy from ~day 21–28 in dogs and cats.
- Differentiates pregnancy from pseudopregnancy.
3. Reference Ranges¶
| Analyte | Species | Reference Range | Unit |
|---|---|---|---|
| T4 (Total) | Canine | 1.0–4.0 | µg/dL |
| T4 (Total) | Feline | 1.0–4.0 | µg/dL |
| TSH | Canine | 0–0.6 | ng/mL |
| Cortisol (basal) | Canine | 2–6 | µg/dL |
| Progesterone (anestrus) | Canine | 0–2 | ng/mL |
| Relaxin (pregnant) | Canine/Feline | > 1.0 | ng/mL |
4. Analytical Performance¶
| Analyte | LOD | Linear Range | Intra-assay CV% | Inter-assay CV% |
|---|---|---|---|---|
| T4 | 0.1 µg/dL | 0.2–10 µg/dL | < 8% | < 10% |
| TSH | 0.03 ng/mL | 0.05–5 ng/mL | < 8% | < 10% |
| Cortisol | 0.2 µg/dL | 0.5–30 µg/dL | < 8% | < 10% |
| Progesterone | 0.2 ng/mL | 0.5–40 ng/mL | < 8% | < 10% |
| Relaxin | 0.1 ng/mL | 0.3–20 ng/mL | < 8% | < 10% |
5. Interpretation Notes¶
- T4 low + TSH high → primary hypothyroidism (canine).
- Progesterone > 2 ng/mL then rapid rise to > 5 ng/mL → ovulation window (breed/AI within 24–48 h).
- Relaxin > 1 ng/mL at day 21–28 → pregnancy confirmed.
- Cortisol should always be interpreted with clinical signs and, where indicated, dynamic testing.
For structured per-analyte data, see the Data Hub. For methodology, see Validation Methodology.
Authored by: Migibio Clinical & Scientific Affairs, Guangzhou Magic Biotech Co., Ltd.
Reviewed by: Migibio R&D Quality Committee
Last updated: 2026-08-13
Disclosure: Migibio (Guangzhou Magic Biotech Co., Ltd.) is the manufacturer of the FIA680/FIA880 analyzers and FICT reagents referenced in this content. See our Editorial & Review Policy.