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Understanding Lab Values > Thyroid Tests

Thyroid Tests

Your thyroid is a bow-tie shaped gland that sits low in your neck. It produces hormones that help your body with metabolism.

The pituitary gland in your head regulates how much hormone the thyroid gland makes through a complex system of feedback regulation. One part of the body senses that a level is high or low, and tells another part of the body to stop making or increase production of various hormones.

Thyroid disease is very common and can begin at any age. Testing for thyroid problems is on all state newborn screens to identify newborns affected with thyroid conditions. It is very important to identify thyroid problems and treat them in young children. If left untreated, mental retardation can result because the developing brain is unable to grow properly. In older children and adults thyroid disease might exist for a while before it is identified.

Symptoms of low thyroid hormone (hypothyroidism)

  • include poor growth in children (weight can increase, but length/height does not keep up)
  • tiredness/fatigue
  • constipation
  • feeling cold
  • weakness
  • dry hair and hair loss
  • muscle cramps
  • depression
  • irritability
  • abnormal menstrual cycles

Symptoms of high thyroid levels (hyperthyroidism)

  • heart palpitations/increased heart rate
  • increased energy or fatigue
  • tremor/hands shaky
  • eyes enlarging out of socket
  • weight loss despite eating well
  • feeling hot/sweaty
  • nervousness
  • insomnia
  • increased bowel movements
  • menstrual cycle changes
  • muscle weakness
  • hair loss

Thyroid hormones include thyroxine (T4) and triiodothyronine (T3). Both of these hormones can be attached to proteins. We typically follow "Free T4" levels, which is T4 not attached to a protein. This is the active thyroid hormone your body uses. When T3 and T4 levels become low, the pituitary gland produces Thyroid Stimulating Hormone (TSH). TSH stimulates the thyroid gland to make more thyroid hormone. When the T3 and T4 levels are normal, the pituitary gland decreases the TSH production.

If any part of the feedback loop stops working, thyroid levels can increase above normal, or decrease below normal. For instance, if the pituitary gland stops working, the TSH will be low, leading to low T3 and T4 levels. If the thyroid gland is unable to make more hormones, the T3 and T4 levels are low, so the pituitary sends out more and more TSH. In this case the TSH level will be elevated, but the T3 and T4 levels low.

If there are abnormal thyroid tests, the first thing to do is to repeat the test. Since this is a feedback system, at any one time a level can be high or low, but the body corrects itself. Sometimes your doctor will order additional tests when repeating these labs. These help to identify the part of the feedback loop that is not working. If repeat testing continues to be abnormal, your physician will most likely want to see your child to begin treatment and discuss the issue in more detail.

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