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Hormone testing guide · 2026
Thyroid testing is simple until someone orders everything at once. TSH first, then add deliberately.
A thyroid function test in Bangkok should start with TSH. Because the pituitary responds to small changes in thyroid hormone with a large change in TSH, it is the most sensitive first-line marker in people with an intact pituitary. Free T4 is added when TSH is abnormal or when pituitary disease is suspected. Free T3 is useful mainly in suspected hyperthyroidism. TPO antibodies help establish an autoimmune cause. One practical warning: high-dose biotin supplements can produce dramatically false results on many immunoassays, and should be stopped for a few days before testing.
| First test | TSH |
|---|---|
| Add free T4 | If TSH is abnormal, or pituitary disease is suspected |
| Add free T3 | Mainly in suspected hyperthyroidism |
| Antibodies | TPO antibodies to establish an autoimmune cause; TRAb in Graves disease |
| Stop before testing | High-dose biotin supplements — usually 2–3 days, on medical advice |
| Price | Listed on the MedEx lab test catalogue — same price nationwide |
The pituitary monitors circulating thyroid hormone and adjusts TSH accordingly. The relationship is logarithmic, so a small fall in thyroid hormone produces a large rise in TSH. That amplification makes TSH the most sensitive single marker of thyroid status.
The exception is pituitary or hypothalamic disease, where TSH may be inappropriately normal or low despite low thyroid hormone. In that setting, free T4 must be measured alongside.
TSH also has a mild diurnal variation, higher at night and in the early morning. This matters mainly when a result sits just outside the reference range — a borderline value is worth repeating at a consistent time before acting on it.
| High TSH, low free T4 | Primary hypothyroidism. Add TPO antibodies to establish an autoimmune cause. |
|---|---|
| High TSH, normal free T4 | Subclinical hypothyroidism. Repeat in 6–12 weeks with TPO antibodies before deciding on treatment. |
| Low TSH, high free T4 or free T3 | Hyperthyroidism. Needs prompt assessment; TRAb and imaging help identify the cause. |
| Low TSH, normal free T4 and free T3 | Subclinical hyperthyroidism. Repeat and assess; more relevant in older adults and those with heart disease. |
| Low TSH, low free T4 | Consider central (pituitary) hypothyroidism or recovery from non-thyroidal illness. |
| Normal TSH, symptoms persist | Look elsewhere — iron deficiency, sleep apnoea, depression, anaemia, poor sleep, uncontrolled glucose. |
Order TSH with free T4 and TPO antibodies from one sample.
Biotin is present in many hair, skin and nail supplements, often at doses far above nutritional requirements. It interferes with the streptavidin-biotin chemistry used in many immunoassays and can produce results that mimic Graves disease — a falsely low TSH with falsely high free T4 and free T3.
Regulators have issued explicit warnings about this. If you take biotin, tell the laboratory, and discuss stopping it for a few days before testing.
Other interferences worth flagging: heterophile antibodies can distort results unpredictably; amiodarone, lithium, iodine-containing contrast and some immunotherapies alter thyroid function directly; acute illness produces a non-thyroidal illness pattern that resolves without treatment, which is why testing during acute illness is discouraged.
Thyroid dysfunction affects ovulation, miscarriage risk and pregnancy outcomes, so TSH and free T4 are standard in any fertility workup. In pregnancy, reference ranges shift — trimester-specific ranges must be used rather than standard adult ranges.
Anyone with known thyroid disease planning pregnancy should have their status reviewed before conceiving, since levothyroxine requirements typically rise in early pregnancy. This is a conversation to have in advance through a specialist teleconsultation.
TPO antibodies are relevant here too: they predict a higher risk of progression to overt hypothyroidism and of postpartum thyroiditis. Related prenatal screening options are covered on the MedEx NIPT service page.
Free T3 earns its place in suspected hyperthyroidism, where T3-predominant disease can occur. In hypothyroidism it adds little to management.
Reverse T3 is widely marketed and not recommended for routine clinical use. It rises in acute illness, starvation and stress as a normal physiological response, and there is no validated treatment threshold based on it.
Thyroglobulin is used for monitoring after thyroid cancer treatment, not for screening.
A sensible standard panel is TSH, free T4 and TPO antibodies, with free T3 added if hyperthyroidism is suspected. Order it alongside ferritin, full blood count and vitamin D within a MedEx checkup package if fatigue is the presenting symptom.
TSH. Because the pituitary amplifies small changes in thyroid hormone into large changes in TSH, it is the most sensitive first-line marker in people with an intact pituitary. Free T4 is added when TSH is abnormal.
No. Fasting is not required, though TSH has a mild daily variation and borderline results are best repeated at a consistent time of day.
Yes, substantially. High-dose biotin can produce a falsely low TSH with falsely raised free T4 and free T3, mimicking Graves disease. Tell the laboratory and discuss stopping it for a few days before testing.
It is not recommended for routine clinical use. Reverse T3 rises in acute illness, starvation and stress as a normal response, and there is no validated treatment threshold based on it.
A raised TSH with a normal free T4. It should be confirmed on a repeat sample 6 to 12 weeks later, with TPO antibodies, before deciding whether treatment is appropriate.
Yes. Trimester-specific reference ranges are used, and levothyroxine requirements typically rise in early pregnancy, so anyone with known thyroid disease should have their status reviewed before conceiving.
Normal thyroid but still tired? A broader panel usually finds the reason.
Medical disclaimer: This article is general health information and does not replace medical advice, diagnosis or treatment. Laboratory reference ranges differ between laboratories and results must be interpreted alongside your symptoms, medicines and medical history. Speak with a qualified clinician before starting, stopping or changing any treatment. Service details, inclusions and prices can change — confirm them with MedEx before booking.
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