A hypothyroid diagnosis is often quickly made from two basic thyroid tests – if your TSH is high and your Free T4 is low. Your doctor might even make a diagnosis solely based on high TSH. If this is your experience, my advice is find a new doctor who is interested in giving you the whole picture and treating you, not your lab tests.
What is TSH?
TSH, or thyroid stimulating hormone, is produced by your pituitary gland (not your thyroid as is commonly assumed) when your circulating levels of T4 (thyroxine) and T3 (triiodothyronine – don’t worry I’ll come to T3) are low. This increase in TSH in your blood then tells your thyroid to produce more T4 and T3, to raise your levels. It’s normal for levels to fluctuate in times of stress, low body temperature, pregnancy and illness.
Why is Free T4 important?
T4 is one of the hormones produced by the thyroid gland which affects the function of nearly every system in your body.
Free T3 – the most important thyroid hormone
But T4 is only one of the two biologically active hormones the thyroid produces. The other is T3 (remember triiodothyronine?). Although T4 is is commonly used to diagnose hypothyroid, it is 300% less biologically active than T3. Also, only 20% of our T3 is produced by the thyroid. The remaining 80% is converted from T4 in organs like the liver and kidney. So these organs need to be healthy for the effective conversion of T4 to T3. It’s important to test for T3 because you may not be converting T4 effectively to the much more usable form, T3.
Also, make sure your doctor is testing for ‘Free’ T4 and ‘Free’ T3, not ‘total’ hormone levels. Total levels measure all the thyroid hormones in your body. ‘Free’ levels measure the hormone actually available to your cells to do its job.
90% of hypothyroid cases are actually caused by an autoimmune reaction in which anti-thyroid antibodies are produced which damage the thyroid. This damage causes it to become under-active or hypothyroid. This is known as Hashimoto’s disease.
Hashimoto’s is diagnosed when one or two types of anti-thyroid antibodies are high. Thyroid peroxidase antibodies (TPO) are more common, but sometimes thyroglobulin antibodies (TGA) are also found. Anti-thyroid antibody screening is crucial in thyroid disease to determine if the cause is autoimmune, as in Hashimoto’s.
No amount of Synthroid or Levothyroxine is going to reduce your thyroid antibodies, which if often the cause of your hypothyroid.
Thyroid function tests I recommend
Don’t just rely on your TSH and T4 – these are the tests you need to see the whole picture.