Thyroid Function Tests

Definition of Thyroid Function Tests

Thyroid function tests are a series of blood tests that are used to measure the functioning of the thyroid gland.

It is mainly helpful in regulating metabolism, mood, and the generation of energy.

Hormones released – Hypothalamus – TRH, Pituitary- TSH, and Thyroid gland – T3 & T4.


The thyroid function tests performed are

  1. T3 – Triiodothyronine.
  2. T4 – Thyroxine.
  3. TSH – Thyroid stimulating hormone.

T3 Triiodothyronine test:

This is a biologically active hormone. It is an autoimmune disorder. Its normal range is 100- 200ng/dl.

This is suggested if T4 & TSH tests suggest hyperthyroidism.

Abnormality indicates Graves’s disease.

T4 Thyroxine test:

The normal range is 50-160 or 9-25 pmol/L. Here T4 gets converted into T3.

Here T4 is of two types one is bound to proteins and the other is free T4.

TSH test:

The normal range is 0.4 – 4.0 mu/L.


  1. A high TSH and low T4 – Hypothyroidism.
  2. A low TSH and high T4 – Hyperthyroidism.
  3. Abnormal TSH with normal T4 – Thyroid disorder.
  4. Low TSH and low T4 – Disorder of the pituitary gland.

Pulmonary Function Tests

  1. Inspiratory capacity: It is the sum of Tidal volume and Inspiratory reserve volume. (500ml+3100ml = 3600 ml)
  2. Residual volume: It is the sum of residual volume and expiratory reserve volume (1200ml + 1200ml = 2400ml).
  3. Vital capacity: It is the sum of Inspiratory reserve volume, tidal volume, and expiratory reserve volume (4800ml).
  4. Total lung capacity: It is the sum of vital capacity and residual volume (4800ml+1200ml = 6000ml).
  5. Tidal volume: It is the amount of air exhaled or inhaled at rest (500 – 750ml).

Tests Associated with Cardiac Disorders

Cardiac biomarkers are the substances that are released into the blood when the heart is damaged or stressed. These tests are mainly of two types.

  1. Laboratory tests
  2. Non-laboratory tests


  1. Troponin test
  2. Creatine test
  3. Myoglobin test

Troponin Test:

Troponin is a protein found in skeletal and contractual fibers of the heart.

Troponin 1 & T are cardiac specific.

Normal range is 0 – 0.3 ng/ml.

Creatine Test:

It is an enzyme that catalyzes the conversion of Creatine to phosphocreatine degrading ATP to ADP. This can be measured as ck-mm, ck-mb, CK- BB.


The normal range is for males-171U/L, and females – 145U/L.

Myoglobin Test:

This test is usually performed to measure the level of protein Myoglobin in blood.

The test is mainly performed to detect muscle damage.

RESULTS: The normal range is 0 – 85 ng/ml.


ECG or EKG test:

It is a tool that is used to measure and record the electrical activity of the heart. It is mainly used to detect defects in the heart like:

  1. Atrial enlargement
  2. Ventricular hypertrophy
  3. Arrhythmia.
  4. The P wave is a record of electrical activity through the upper heart chamber.
  5. The QRS complex is the movement of electrical impulses through the lower ventricles.
  6. The ST segment appears as a straight line between QRS and T wave. This shows ventricle is contracting with no electrical flow.
  7. The T wave shows when ventricles are setting for the next muscle.

Chest X-Ray:

  • It is a non-invasive method that is used to record medical imaging.
  • It is mainly used to check the enlargement of the heart.

CT Scan:

it is a scan that combines many X-ray images with the help of a computer to generate cross-sectional views of the body.

It is mainly used to evaluate:

  1. The heart muscle.
  2. The coronary arteries
  3. The pulmonary veins
  4. Pericardium.

MRI Scan:

This is useful for detecting structural problems with the heart such as congenital heart disease, function of heart valves, etc.

Cardiac Catheterization or Coronary Angiogram:

Here a catheter is placed through the IV and into the blood vessel in the leg or neck. In the test, a contrast dye is injected through the catheter so that x-ray valves, coronary arteries, and heart chambers are clearly seen in the x-ray machine. The test may last for 30-60 min.

This is mainly used to examine

  • Measure pressure and blood flow in chambers and in large arteries of the heart.
  • Measure the oxygen concentration.

Tread Mill Test:

  • The stress test is mainly used to assess the heart condition during exercise when the demand for oxygen increases.
  • It measures the patient’s responses to workload.
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