Differentiating Hypothyroid Types

The blood chemistry screen and thyroid panel can be used to differentiate the different types of hypothyroidism.

Primary hypothyroidism is the classic hypothyroid condition. The locus of the problem lies in the thyroid itself. As such the output of T4 from the thyroid is low and the conversion of T4 into T3 is also decreased. On a blood test you will see high TSH, and low T4 and T3 levels.

Secondary hypothyroidism is a form of hypothyroidism is becoming more common. The thyroid itself is working normally, but the anterior pituitary is no longer able to produce TSH due to some kind of anterior pituitary dysfunction. on a blood test you will see low TSH levels, normal T4 and T3 levels.

Tertiary hypothyroidism is not so common and is caused by an inadequate secretion of TRH from the hypothalamus causing a decreased TSH level.

How do you differentiate between these three types of hypothyroidism?

A decreased TSH level may be due to an inadequate secretion of Thyroid Releasing Hormone (TRH) from the hypothalamus. Clinically an intravenous injection of TRH is administered to try to stimulate the production of TSH from the pituitary.

1. In primary hypothyroidism there is an exaggerated TSH response after administration of TRH.
2. In secondary hypothyroidism due to anterior pituitary hypofunction there is no significant rise in TSH levels after administration of TRH.
3. In tertiary hypothyroidism due to a hypothalamic hypofunction there will be a delayed rise in TSH of approximately 30 minutes after administration of TRH.

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Dr. Dicken Weatherby is a graduate of the National College of Naturopathic Medicine in Portland, Oregon. He is the co-author of the best-selling book "Blood Chemistry and CBC Analysis- Clinical Laboratory Testing from a Functional Perspective".

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