Pre test Information : No Special Preparation Needed

TSH, T4, and occasionally T3 are the most common blood tests for thyroid function.

In most cases, your TSH level will be the first hormone measured, because if it is normal, your thyroid is most likely functioning normally. The only exception is when a pituitary problem is suspected, in which case T4 should also be measured.

Doctors should measure T4 in addition to TSH in children because T4 is essential for brain development.

Many laboratories use a 'cascade' system, which means that if the TSH is abnormal, the other hormones will be measured.

• If the TSH level is above the reference range, the blood T4 level will be measured.
• If the TSH level is below the reference range, the blood T4 and T3 levels will be measured.

A blood sample is drawn from a vein in the arm and sent to a laboratory for analysis. Typically, the 'free' or active portion of T4 and T3 is measured (i.e., FT4 and FT3).

When should I have a thyroid function blood test?
If you have any of the following symptoms, you should see your doctor and request a blood test:

• Symptoms of an overactive or under-active thyroid
• Neck swelling or thickening
• An irregular or rapid heart rate
• High cholesterol (which causes atherosclerosis – a build-up of fat in the arteries)
• Osteoporosis (fragile or thinning bones)
• Fertility issues, irregular menstrual cycles, recurrent miscarriages, and low libido
• A family history of autoimmune disorders, such as type 1 diabetes, vitiligo, and so on,

or if you are

• Feeling ill after having a baby
• Planning a pregnancy or already pregnant? (and you have a family history or personal history of thyroid disorders, a past history of postpartum thyroiditis, or type 1 diabetes)

If you:

• You have a thyroid disorder.
• You've previously been treated for an overactive thyroid (radioactive iodine, thyroid surgery, medication)
• You received head and neck irradiation following surgery for head and neck cancer.
• Before starting treatment with amiodarone, lithium, or alemtuzumab, then 6-12 months during treatment, and 12 months after treatment

People with Down syndrome, Turner syndrome, Addison's disease, or other autoimmune diseases should also be tested on a regular basis.

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