Thyroid Function Test

Thyroid function tests measure how well your thyroid gland is working. The thyroid is a small, butterfly-shaped gland in your neck that produces hormones that regulate your metabolism, energy, and many body functions.

These tests are used to diagnose thyroid disorders like hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), monitor treatment effectiveness, and screen for thyroid problems. Thyroid disorders are common and easily treatable once diagnosed.

Thyroid function tests measure:

• TSH (Thyroid-Stimulating Hormone): Primary test, from pituitary gland
• T4 (Thyroxine): Main thyroid hormone
• T3 (Triiodothyronine): Active thyroid hormone
• Thyroid antibodies: If autoimmune cause suspected
• Reverse T3: Sometimes measured

Why It Matters:

• TSH is usually the first test
• High TSH = hypothyroidism (underactive)
• Low TSH = hyperthyroidism (overactive)
• Guides diagnosis and treatment

Usually minimal preparation needed:

• You can eat and drink normally
• Take medications as usual (unless told otherwise)
• Some medications may affect results (tell your doctor)
• No special diet needed
• No fasting required (usually)

Before the Test:

• Inform doctor of all medications
• Some medications affect thyroid function
• May need to adjust timing of medications
• Follow specific instructions

Important: Some medications and supplements can affect results. Always tell your doctor about everything you take.

During the Test:

1. You'll sit or lie down
2. A tourniquet is applied to your upper arm
3. The area is cleaned with antiseptic
4. A needle is inserted into a vein
5. Blood is collected
6. The needle is removed and pressure applied
7. A bandage is placed over the site

After the Test:

• Keep the bandage on for a few hours
• Avoid heavy lifting with that arm
• Minor bruising is normal
• Results usually available in 1-3 days
• Your doctor will contact you with results

Normal Ranges (may vary by lab):

TSH:

• Normal: Usually 0.4-4.0 mIU/L
• May vary slightly by lab
• Optimal range may be narrower

T4:

• Normal: Usually 4.5-11.2 mcg/dL
• Normal range varies

T3:

• Normal: Usually 100-200 ng/dL
• May vary by lab

Interpretation:

Hypothyroidism (Underactive):

• High TSH
• Low T4
• Symptoms: fatigue, weight gain, cold intolerance

Hyperthyroidism (Overactive):

• Low TSH
• High T4 and/or T3
• Symptoms: weight loss, rapid heartbeat, anxiety

Subclinical:

• TSH slightly abnormal
• T4 normal
• May or may not need treatment

Important: Your doctor will interpret results in context of your symptoms, medical history, and other factors. Normal ranges can vary.

• Results outside normal range
• Symptoms of thyroid problems
• Need to adjust thyroid medication
• Questions about results
• Need for follow-up testing
• Planning pregnancy (if thyroid issues)

• Results within normal range
• Understanding your results
• Following treatment plan
• Regular monitoring scheduled

If Hypothyroidism:

• Thyroid hormone replacement (levothyroxine)
• Take daily, usually in morning
• Regular monitoring
• Usually lifelong treatment

If Hyperthyroidism:

• Medications to reduce hormone production
• Radioactive iodine (in some cases)
• Surgery (in some cases)
• Regular monitoring

Monitoring:

• Regular blood tests
• Usually every 6-12 weeks initially
• Then every 6-12 months once stable
• May need dose adjustments

Most thyroid problems cannot be prevented, but:

• Adequate Iodine: Usually not a problem in developed countries
• Regular Check-ups: Especially if at risk
• Know Family History: Thyroid disease can run in families
• Early Detection: Get tested if symptoms develop
• Follow Treatment: If diagnosed, take medications as prescribed

Normal Thyroid Function:

• Pituitary gland produces TSH
• TSH tells thyroid to produce T4 and T3
• Thyroid hormones regulate metabolism
• Feedback loop controls levels

Hypothyroidism:

• Thyroid doesn't produce enough hormones
• TSH increases (trying to stimulate thyroid)
• Metabolism slows down
• Various symptoms develop

Hyperthyroidism:

• Thyroid produces too much hormone
• TSH decreases (trying to slow thyroid)
• Metabolism speeds up
• Various symptoms develop

Causes:

• Autoimmune: Hashimoto's (hypo), Graves' (hyper)
• Medications: Some affect thyroid
• Radiation: Can damage thyroid
• Surgery: Removal of thyroid
• Other: Various causes

• More common in women
• May develop after pregnancy
• May affect menstrual cycles
• Important during pregnancy

• More common with age
• Symptoms may be mistaken for aging
• May need medication adjustments
• Regular monitoring important

• Important to monitor during pregnancy
• May need dose adjustments
• Affects baby's development
• Regular testing needed

If you have thyroid disease, usually every 6-12 weeks initially, then every 6-12 months once stable. If you don't have thyroid disease, testing depends on symptoms and risk factors.

Yes. Some medications, including thyroid medications, can affect results. Always tell your doctor about all medications you take. Take thyroid medication as directed (usually morning, before eating).

TSH is produced by the pituitary gland and tells the thyroid to work. T4 is produced by the thyroid. High TSH usually means the thyroid isn't working well (hypothyroidism). Low TSH usually means the thyroid is overworking (hyperthyroidism).

Most thyroid problems cannot be prevented, but early detection and treatment can prevent complications. If you have symptoms, get tested.

For hypothyroidism, usually yes. Most people need to take thyroid medication for life. For hyperthyroidism, treatment depends on the cause and may be temporary or permanent.