Thyroid Physiology

The thyroid gland is located in the lower front of the neck; it produces thyroid hormone which helps control many of our body’s functions (see Thyroid Disease for a discussion of Hypothyroidism and Hyperthyroidism).ย  But the blood tests we use may be a little confusing, since the main one is high in Hypothyroidism, and low in Hyper- (!!!).ย  So here’s an explanation of how thyroid hormone is produced, & what the tests measure.

1.  A part of the brain called the Hypothalamus produces a hormone that it sends to the Pituitary Gland (located at the base of the brain, but somewhat separate from it), telling it to stimulate the Thyroid Gland.

2. The Pituitary Gland produces its Thyroid-Stimulating Hormone (TSH), that tells the Thyroid Gland to work harder.

3.  The Thyroid Gland produces its two hormones, thyroxine (T4) and triiodothyronine (T3) which travel throughout the body to many organs.

4.  The thyroid hormones T4 and T3 also travel to the brain.  There, the Hypothalamus and Pituitary sense there’s plenty available, so they slow down, and produce less TSH.

5.  Less TSH means the Thyroid Gland slows down.  So there’s less T4 and T3.  Then the Hypothalamus and Pituitary rev up TSH production.  Etc. etc., occurring every moment of our lives.

Therefore, we diagnose Hypothyroidism by a high TSH.  That’s because if a diseased Thyroid Gland can’t produce, the Pituitary churns out more and more TSH (in vain).  The T4 and T3 are low, but the TSH is a better test.

Conversely, we diagnose Hyperthyroidism when there’s no TSH in the blood.  That’s because an overactive Thyroid is making too much thyroid hormone, so the Pituitary shuts down.  The T4 and T3 are high, but the TSH is a better test.

To summarize:

  • Hypothyroidism has high TSH, low T4 and T3
  • Hyperthyroidism has low (or no) TSH, high T4 and T3

Both Hypothyroidism & Hyperthyroidism can also be โ€œsubclinical,โ€ when the TSH is low or high, but the thyroid hormones T4 and T3 are normal.  The subclinical forms often donโ€™t need treatment.

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