Thyroid Function Tests: Your Complete Guide
An estimated 1 in 8 people will develop a thyroid condition during their lifetime. Understanding your thyroid blood tests is the first step to staying ahead of it.
What Is Your Thyroid?
Your thyroid is a small, butterfly-shaped gland at the base of your neck. Despite weighing only about 20 grams, it acts as your body's thermostat, controlling how fast every cell in your body works. It influences your heart rate, body temperature, metabolism, mood, energy levels, and even how quickly you think.
The Thyroid Feedback Loop
Your thyroid is controlled by a precise feedback system. The hypothalamus (in your brain) releases TRH, which tells the pituitary gland to release TSH. TSH then stimulates the thyroid gland to produce T4 and T3 hormones. When T3 and T4 levels rise high enough, they signal the pituitary to reduce TSH production, completing the loop. This is why TSH moves in the opposite direction to thyroid hormones: high TSH usually means low thyroid function, and vice versa.
When T3/T4 levels are sufficient, they signal the pituitary to slow down TSH (negative feedback).
The Key Markers
A standard thyroid panel includes several markers that work together to paint a complete picture of your thyroid health. Here are the four most important ones your doctor will look at.
TSH
The single most important screening test for thyroid function. Produced by the pituitary gland, TSH tells the thyroid how much hormone to make. It is the first marker to change when thyroid function shifts.
Normal Range: 0.4 - 4.0 mIU/L
High: Suggests underactive thyroid (hypothyroidism). The pituitary is working harder to stimulate a sluggish thyroid.
Low: Suggests overactive thyroid (hyperthyroidism). The pituitary is backing off because thyroid hormone levels are already high.
Free T4
The main hormone produced by the thyroid gland. T4 circulates in the blood and is converted to active T3 in tissues. 'Free' means the unbound, biologically available portion (about 0.03% of total T4).
Normal Range: 10 - 20 pmol/L (0.8 - 1.8 ng/dL)
High: Excess thyroid hormone production, seen in hyperthyroidism or overmedication with levothyroxine.
Low: Insufficient thyroid hormone, seen in hypothyroidism. May cause fatigue, weight gain, and cold intolerance.
Free T3
The active thyroid hormone that directly affects cells. About 80% of T3 is produced by converting T4 in peripheral tissues (liver, kidneys, muscles). T3 is 3 to 5 times more potent than T4.
Normal Range: 3.5 - 6.5 pmol/L (2.3 - 4.2 pg/mL)
High: Often elevated in hyperthyroidism, especially T3 thyrotoxicosis where T3 rises before T4.
Low: May indicate poor T4-to-T3 conversion (seen with selenium deficiency, chronic illness, or certain medications).
Thyroid Antibodies
Anti-TPO (thyroid peroxidase) and anti-Tg (thyroglobulin) antibodies indicate autoimmune thyroid disease. Present in up to 95% of Hashimoto's patients and 50-80% of Graves' disease patients.
Normal Range: TPO-Ab: < 35 IU/mL | TgAb: < 40 IU/mL
High: Autoimmune thyroid disease (Hashimoto's or Graves'). High TPO antibodies increase the risk of future hypothyroidism, even if current thyroid levels are normal.
Low: Normal / no autoimmune thyroid component detected.
Hypothyroid vs Hyperthyroid
Thyroid conditions fall into two broad categories based on whether the gland is producing too little or too much hormone. The symptoms, lab patterns, and treatments differ significantly between the two.
Lab Pattern
Common Symptoms
Everything slows down when your thyroid is underactive:
Fatigue and sluggishness, even after adequate sleep
Unexplained weight gain or difficulty losing weight
Cold intolerance - always needing an extra layer
Constipation and slowed digestion
Dry skin, brittle nails, and thinning hair
Depression, low mood, and difficulty concentrating (brain fog)
Muscle aches and joint stiffness
Elevated cholesterol levels
Menstrual irregularities (heavy or prolonged periods)
Common Causes
- Hashimoto's thyroiditis
The most common cause worldwide. An autoimmune condition where the immune system gradually destroys thyroid tissue.
- Iodine deficiency
Still the leading cause in developing countries. Iodine is essential for thyroid hormone production.
- Previous thyroid surgery or radioactive iodine treatment
Partial or total thyroid removal reduces hormone production capacity.
- Medications
Lithium, amiodarone, interferon-alpha, and certain immunotherapy drugs can impair thyroid function.
Treatment Overview
The standard treatment is daily oral levothyroxine (synthetic T4), which replaces the missing hormone. Dosing is individualised based on weight, age, and TSH levels, then adjusted over weeks to months until TSH stabilises within the target range (typically 0.5-2.5 mIU/L for most adults). Levothyroxine should be taken on an empty stomach, 30-60 minutes before breakfast, and away from calcium or iron supplements which can block absorption.
Understanding Subclinical Thyroid Disease
“Subclinical” means the TSH is abnormal but the thyroid hormones (Free T4, Free T3) are still within the normal range. Many patients have no obvious symptoms, yet these early shifts can predict future thyroid problems.
Who Should Get Tested?
Thyroid screening is not routinely recommended for the general population, but certain groups are at significantly higher risk. Talk to your doctor about thyroid testing if any of the following apply to you.
| Who | Recommendation |
|---|---|
| Women over 35 | Screen every 5 years (American Thyroid Association). Women are 5-8x more likely to develop thyroid disease than men. |
| Pregnant women | Test TSH in the first trimester. Untreated hypothyroidism raises the risk of miscarriage, preeclampsia, and impaired foetal brain development. |
| Family history of thyroid disease | First-degree relatives of patients with Hashimoto's or Graves' disease should be tested, even without symptoms. |
| Type 1 diabetes or other autoimmune conditions | Annual thyroid screening recommended. Autoimmune conditions frequently cluster together. |
| History of head or neck radiation | Radiation therapy (e.g. for lymphoma or head/neck cancer) can damage thyroid tissue. Annual monitoring recommended. |
| Taking lithium or amiodarone | Both medications have well-documented effects on thyroid function. TSH should be checked before starting and every 6-12 months during treatment. |
| Adults over 60 | Subclinical hypothyroidism prevalence increases with age. Symptoms may be subtle and attributed to ageing. |
Factors That Affect Your Results
Thyroid test results can be influenced by several factors beyond your actual thyroid function. Being aware of these helps you and your doctor interpret your results accurately.
Time of Day
TSH follows a circadian rhythm, peaking between 2:00-4:00 AM and reaching its lowest point in the late afternoon (around 4:00-6:00 PM). Morning blood draws typically show TSH levels 20-50% higher than afternoon draws. For consistent monitoring, always have your blood drawn at a similar time of day, ideally in the early morning before 10:00 AM.
Biotin Supplements
Biotin (vitamin B7), commonly found in hair, skin, and nail supplements at doses of 5,000-10,000 mcg, can cause significant interference with thyroid immunoassays. It can falsely lower TSH and falsely elevate Free T4 and Free T3, mimicking hyperthyroidism on paper. Stop biotin supplements at least 48-72 hours before thyroid blood work.
Medications
Several medications can alter thyroid test results without affecting actual thyroid function. Glucocorticoids (prednisone) and dopamine suppress TSH. Oestrogen (oral contraceptives, HRT) raises total T4 by increasing binding proteins (Free T4 remains normal). Metformin can lower TSH in diabetic patients. Always tell your doctor about all medications and supplements before testing.
Pregnancy
Pregnancy significantly changes thyroid reference ranges. In the first trimester, hCG stimulates the thyroid, naturally lowering TSH (often to 0.1-2.5 mIU/L). Trimester-specific ranges should be used: 1st trimester TSH 0.1-2.5, 2nd trimester TSH 0.2-3.0, 3rd trimester TSH 0.3-3.5 mIU/L. Total T4 rises due to increased thyroxine-binding globulin, but Free T4 should remain within normal limits.
Monitor Your Thyroid Levels Over Time
Upload your blood test results to SmarterBlood and track TSH, Free T4, Free T3, and thyroid antibodies with interactive graphs. See trends, get AI-powered insights, and share professional reports with your doctor.
This guide is for educational purposes only and does not constitute medical advice. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your results. See our medical disclaimer for more information.