🦋 How to Prepare for a
Thyroid Function Test (TFT)
A thyroid function test (TFT) is a blood test that measures TSH, free T4, and free T3, the hormones that control your body's metabolism. It diagnoses underactive thyroid (hypothyroidism), overactive thyroid (hyperthyroidism), and monitors thyroid conditions on treatment. Minimal preparation is needed, but a few specific points ensure an accurate result.
✅ Preparation Steps
1
No fasting is required
A TFT blood draw does not require fasting. Eat and drink normally before your appointment. The test can be done at any time of day, though early morning is preferred for consistency (TSH has a natural diurnal variation. It peaks in the early hours and is slightly lower by afternoon).
2
If you are on levothyroxine: take it AFTER the blood draw
Levothyroxine (Eltroxin, Synthroid) causes a transient spike in free T4 for about 6 hours after ingestion. Taking your dose before the test will show an artificially elevated free T4, potentially suggesting over-treatment. For an accurate baseline, take your levothyroxine tablet after the blood is drawn on the morning of the test, not before. This is especially important during dose-adjustment monitoring.
3
Stop biotin (vitamin B7) supplements for at least 48 hours before the test
High-dose biotin (common in hair, skin, and nail supplements) interferes with many immunoassay-based thyroid tests, producing falsely low TSH and falsely high free T4, a pattern mimicking hyperthyroidism. If you take biotin supplements (especially doses above 1000 mcg), stop them 48 hours before your TFT.
4
Tell us about all thyroid-related medications
In addition to levothyroxine: carbimazole and propylthiouracil (for hyperthyroidism), amiodarone (heart medication, significantly alters thyroid function), lithium, and interferon are all medications that affect thyroid hormone levels. List all medications when registering.
5
Avoid thyroid massage or neck procedures immediately before the test
Any significant pressure on the thyroid gland (e.g., vigorous neck massage) can transiently release thyroid hormone into the circulation. Avoid physiotherapy or massage targeting the anterior neck in the hours before a TFT.
🎒 What to Bring
- 📋 Prescription or referral for the specific thyroid tests required (TSH only, or full TFT: TSH + free T4 + free T3)
- 📋 Previous TFT results for comparison: trend and baseline are essential for interpretation
- 📋 Complete list of medications including supplements (especially biotin, amiodarone, and any thyroid medications)
- 📋 If monitoring thyroid cancer treatment: bring your recent thyroglobulin results for context
🔬 What to Expect
Registration (2 min)
Confirm which thyroid tests are ordered. Our nurse will check your medication list, specifically asking about levothyroxine timing and biotin supplements.
Blood draw (2–3 min)
A single blood tube is taken from the arm. TSH, free T4, and free T3 are measured from the same tube. For thyroid antibody testing (anti-TPO, anti-TG, or TRAb), an additional tube may be needed.
Results (24 hours)
TSH results are typically available within 24 hours; full thyroid panels including antibodies may take up to 48 hours depending on which tests are requested. We will notify you when results are ready.
🏠 After Your Test
- Take your levothyroxine immediately after the blood draw if you withheld it that morning.
- Resume biotin supplements once the test is complete.
- Resume all normal activities immediately: a TFT is a standard blood draw with no recovery needed.
- Do not attempt to self-interpret your TSH result without knowing your free T4 and clinical context. A TSH of 0.1 mIU/L could indicate genuine hyperthyroidism or simply that you are on a slightly too-high dose of levothyroxine, the treatment implications are completely different.
- If your TSH is abnormal, your doctor will advise on the next steps: dose adjustment (for those on levothyroxine), further antibody testing, thyroid ultrasound, or specialist referral.
- For stable hypothyroidism well-controlled on levothyroxine, annual TSH monitoring is usually sufficient, more frequent testing is only needed after a dose change (recheck 6–8 weeks after adjustment).
Frequently Asked Questions
What does a high TSH mean?
TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland to tell the thyroid to produce more hormone. A high TSH means the pituitary is working overtime to compensate for a thyroid gland that is underperforming. This is hypothyroidism (underactive thyroid). The higher the TSH, generally the more severe the underactivity. A TSH above 10 mIU/L in a symptomatic patient almost always warrants levothyroxine treatment.
Can I take my thyroid medication before the test?
For levothyroxine (hypothyroidism treatment): ideally no, take it after the blood draw to avoid a false elevation in free T4. For carbimazole or propylthiouracil (hyperthyroidism treatment): yes, take as normal. These do not cause the same transient spike issue. When in doubt, contact your doctor the evening before to confirm.
Do thyroid results change throughout the day?
Yes, TSH has a circadian rhythm. It is highest between midnight and 4 am, and progressively lower through the morning and into the afternoon. For consistent monitoring, particularly when adjusting levothyroxine dose, try to have all blood draws at the same time of day, preferably early morning. A TSH of 2.5 at 8 am and a TSH of 1.8 at 3 pm from the same person may simply reflect natural variation rather than a treatment change.
My TSH is within the normal range but I still feel tired, why?
The reference range for TSH (typically 0.4–4.0 mIU/L in most labs) covers 95% of the healthy population, but this does not mean every point within the range feels the same to every individual. Some patients on levothyroxine feel best with a TSH in the lower half of the range (1.0–2.0). Additionally, other causes of fatigue may be present simultaneously, low ferritin, vitamin B12 or D deficiency, depression, sleep apnoea, or anaemia, all of which require their own testing. A comprehensive fatigue screen alongside the TFT is often warranted.
Ready to book your Thyroid Function Test (TFT)?
Lab tests are walk-in; for scans and consultations, book ahead or message us to confirm your appointment. We'll guide you through everything.
💬 Book on WhatsAppLast updated 16 October 2026.