Thyroid lab timing: when to test TSH, free T4 and T3

Want thyroid blood work that actually reflects your body, not your coffee or morning pill? Timing matters. Small daily and medication-related swings can change results enough to affect treatment, so a few simple rules make your labs more useful.

Best time of day to draw thyroid tests

TSH follows a daily rhythm. It tends to be higher overnight and in the early morning and lower in the afternoon. For consistency, many clinics ask for blood draws in the morning. If you always test in the morning, your results are easier to compare over time.

Free T4 and free T3 don’t fluctuate as wildly, but they can still change after you take thyroid medicine. If you take levothyroxine (T4), get your blood drawn before you take your morning dose. If you’re on liothyronine (T3) or a thyroid product that contains T3, timing is different: T3 peaks a few hours after a dose. For accurate T3 levels, aim for either a trough (right before your dose) or a consistent time after dosing—ask your clinician which they prefer.

Medication, supplements and how often to test

Hold or separate interfering substances. Take levothyroxine after the blood draw, or wait at least 4 hours after taking it to test. Iron, calcium, and some antacids can lower absorption of thyroid pills—don’t take them within 4 hours of your thyroid dose or your lab draw.

Biotin (high-dose vitamin B7) can break many lab tests and give wrong thyroid results. Stop biotin at least 48–72 hours before blood work.

Certain medicines (steroids, dopamine, amiodarone) and acute illness can change thyroid numbers. If you’re sick or on new meds, tell the lab or your clinician; they may want to delay testing.

When to repeat tests: after starting or changing levothyroxine, wait about 6–8 weeks before checking TSH. Free T4 can change sooner, but TSH needs weeks to stabilize. Once your dose is stable and you feel well, many people check TSH every 6–12 months or as symptoms dictate. If you’re treating hyperthyroidism, your doctor may follow free T4 and T3 more often.

Practical tips: schedule morning appointments, skip your thyroid pill until after the draw, stop biotin for a few days, separate calcium/iron by 4 hours, and tell the lab about other meds. If your results don’t match how you feel, ask about assay interference or repeat testing at a specific time.

Timing won’t fix every abnormal number, but it reduces false swings and makes decisions clearer. Want a one-line checklist to take to the lab? Morning draw, no thyroid pill first, no biotin for 48–72 hours, separate iron/calcium—done.