TSH and Thyroid Testing Explained
TSH alone screens the whole thyroid axis. Here is how to know when your fatigue is actually thyroid — and when it is not.
How the thyroid axis works
The pituitary releases TSH (thyroid-stimulating hormone) to tell the thyroid gland to produce T4 and T3. When circulating T4/T3 fall, TSH rises; when they rise, TSH falls. This tight feedback loop is why TSH alone is the single most useful screening test for thyroid function.
Normal TSH in most Quebec laboratories: 0.35–4.5 mIU/L. Patterns: high TSH with low T4 = primary hypothyroidism; low TSH with high T4 = hyperthyroidism; abnormal TSH with normal T4 = subclinical.
When to add free T4, free T3, and antibodies
Free T4 (fT4) is added whenever TSH is outside the reference range, when a pituitary problem is suspected, or when monitoring treated hypothyroidism. Free T3 (fT3) is added for suspected hyperthyroidism or in specific complex cases.
Thyroid antibodies (anti-TPO, anti-TG) confirm autoimmune thyroiditis (Hashimoto's) and predict progression risk. TRAb antibodies are used to confirm Graves' disease.
Timing and interference
TSH follows a diurnal rhythm, peaking overnight. For consistency, aim for the same time of day when tracking over time — morning is typical. Biotin supplements (over 5 mg/day, sometimes in hair/nail formulas) can falsely shift TSH results; stop for 48–72 hours before the draw.
Add TSH to your requisition
Ready to book? Reserve a private blood collection appointment on our Book Blood Collection page, or request a private requisition through blood test prescription.
Frequently asked questions
Do I need to fast for TSH?+
No. But being consistent about time of day matters more than fasting.
Can subclinical hypothyroidism cause symptoms?+
Sometimes — treatment decisions weigh symptoms, antibody status, and TSH trend rather than a single number.