Learn what thyroid function tests measure, how they are interpreted, and what abnormal results may indicate about thyroid health.
By Dr. Sanjay Kumar Thakur · Published June 2026 · 8 min read
Editorial note: general health information; seek professional assessment for personal medical decisions.
Thyroid function tests are blood tests that evaluate how well the thyroid gland is working. The thyroid is a butterfly-shaped gland in the neck that produces hormones regulating metabolism.
Thyroid-stimulating hormone (TSH) is the most sensitive test for thyroid function. The pituitary gland produces TSH to signal the thyroid to produce more hormones.
High TSH indicates the pituitary is working hard to stimulate an underactive thyroid, suggesting hypothyroidism. Low TSH suggests the thyroid is overactive, indicating hyperthyroidism.
TSH is typically measured first when thyroid dysfunction is suspected. Normal TSH ranges vary by laboratory but generally fall between 0.5 and 4.5 mIU/L.
Free T4 (thyroxine) is the main hormone produced by the thyroid gland. It circulates in the blood, with only a small free fraction available for use by body tissues.
Low free T4 with high TSH confirms primary hypothyroidism. High free T4 with low TSH confirms hyperthyroidism. Free T4 is less affected by medications and binding proteins than total T4.
Free T3 (triiodothyronine) is the active form of thyroid hormone. Most T3 is produced by converting T4 in body tissues outside the thyroid gland.
T3 testing is useful when hyperthyroidism is suspected but TSH and T4 are normal. T3 is elevated earlier in hyperthyroidism and is more sensitive for Graves disease.
Thyroid antibodies including thyroid peroxidase antibodies (TPO) and thyroglobulin antibodies (TgAb) indicate autoimmune thyroid disease, such as Hashimoto thyroiditis or Graves disease.
Thyroid function tests help diagnose hypothyroidism, hyperthyroidism, and monitor treatment. Regular testing is important for anyone taking thyroid medication.
Abnormal results should always be discussed with a healthcare provider who can correlate lab values with clinical symptoms and recommend appropriate treatment.
Using this guide safely
Learn what thyroid function tests measure, how they are interpreted, and what abnormal results may indicate about thyroid health. This article gives general context for a conversation with a qualified clinician; it is not a diagnosis, prescription, or personal treatment plan. Screening, testing, medicines, and lifestyle advice should be matched to age, symptoms, pregnancy status, medical history, medicines, and local availability.
How a clinician may assess the concern
A clinician would usually check what changed, when it began, what makes it better or worse, relevant diagnoses, medicines and supplements, allergies, family history, and previous results. The goal is to decide whether self-care, routine review, testing, referral, or urgent care is the safest next step.
Questions worth asking
- What are the likely explanations and important alternatives?
- Is a test, medicine, or procedure needed now, and what would it change?
- What are the benefits, limitations, risks, costs, and consequences of waiting?
- Which warning signs need urgent care, and when should progress be reviewed?
Results, follow-up, and local access
Ask who will explain a result or review the response to treatment, how long follow-up usually takes, and what to do if the plan does not help. In Nepal, availability, cost, language, travel distance, and access to a qualified clinician may affect the best next step. Keep copies of reports and use them during follow-up rather than repeating information from memory.
When to seek professional advice
Seek urgent care for severe weakness, confusion, chest symptoms, severe dehydration, pregnancy with concerning symptoms, or very abnormal readings.
Preparing for a consultation
Bring lab reports, medicine doses, symptom diary, relevant menstrual or pregnancy history, and prior imaging.
Common questions
What does this article cover?
Learn what thyroid function tests measure, how they are interpreted, and what abnormal results may indicate about thyroid health. It is general health information and does not diagnose an individual condition.
When should I contact a health professional?
Seek urgent care for severe weakness, confusion, chest symptoms, severe dehydration, pregnancy with concerning symptoms, or very abnormal readings. For urgent symptoms or safety concerns, use emergency or urgent-care services rather than waiting for a routine appointment.
How should I use this information before an appointment?
Bring lab reports, medicine doses, symptom diary, relevant menstrual or pregnancy history, and prior imaging.
Category: Endocrinology
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