Why it is performed
- Detect hypothyroidism or hyperthyroidism.
- Monitor levothyroxine or antithyroid treatment.
- Assess fatigue, weight changes, palpitations, menstrual changes or goitre.
- Use pregnancy-specific intervals.
How it is performed
TSH is the usual first-line test. If abnormal, free T4 helps distinguish overt hypothyroidism or hyperthyroidism from subclinical forms.
Preparation
- High-dose biotin can interfere with some immunoassays; report hair supplements or multivitamins.
- Take levothyroxine consistently and follow instructions on whether to take it before or after collection.
- Report pregnancy, acute illness and treatment such as amiodarone, lithium or steroids.
How Askabide processes it
Combined pathway
TSH and free T4 may be measured by fluorescence immunoassay on ichroma II when validated cartridges are available. The laboratory must apply method-specific intervals and internal controls.
Part is performed at Askabide and another phase or confirmation is processed externally.
Results and indicative intervals
Intervals vary by method, analyser, reagent, units, age, sex, pregnancy, menstrual-cycle phase and reference population. The valid interval is the one printed on the laboratory report.
| Parameter or result | Indicative reference | How it is interpreted |
|---|---|---|
| Adult TSH | ≈0,4–4,0 mUI/L | The laboratory interval and pregnancy-specific range take precedence. |
| Free T4 | ≈0,8–1,8 ng/dL | Interpret with TSH; units and method change the interval. |
| High TSH + low FT4 | Pattern compatible with primary hypothyroidism | Requires clinical assessment and often confirmation. |
| Low TSH + high FT4 | Pattern compatible with hyperthyroidism | May require T3, antibodies and imaging depending on the case. |
Important limitations
- Acute non-thyroidal illness may temporarily alter results.
- Pregnancy requires trimester-specific intervals.
- Biotin may cause misleading results in some assays.
- A single result should not trigger dose changes without advice.