Thyroid-stimulating hormone (TSH) was supposed to be a simple way to detect thyroid disease. Before its invention in the 1970s, doctors diagnosed thyroid disease by symptoms.
TSH is the pituitary’s message to the thyroid to make more hormone. When the pituitary detects a drop in thyroid hormone, it makes more TSH. So—in theory—a normal TSH should mean that there is adequate thyroid hormone in the body. But there’s a problem: The pituitary responds differently to thyroid hormone than does the rest of the body.
That’s because enzymes (D2 deiodinases) in the pituitary are more efficient at T4 → T3 activation than the enzymes that are found everywhere else (D1 deiodinases). The result is that under certain circumstances (see below), the pituitary will be tricked into thinking there enough thyroid hormone when there really isn’t. Therefore TSH will remain low.
When are thyroid tests unreliable?
- Inflammation. Inflammatory cytokines actively suppress both TSH and thyroid hormones T4 and T3. This can be caused by acute and chronic illness, infection, smoking, and even autoimmune thyroid disease. (It’s also called euthyroid sick syndrome or low T3 syndrome.)
- Stress. Cortisol suppresses the release of TSH from the pituitary. (Corticosteroid steroid medication like prednisone does the same thing.)
- Insulin resistance. Insulin upregulates D2 deiodinase and T3 at the pituitary—but decrease T3 everywhere else.
- Dysbiosis. Imbalance of the intestinal bacteria.
- Years on standard thyroid medication (thyroxine or T4), because more of it is converted into reverse T3.
- Iron deficiency.
- Protein deficiency.
- Low-calorie dieting. (That’s why dieters have a slow metabolism.)
- Environmental toxins.
- Hormonal birth control and other forms of HRT.
- Advancing age.
Not all of these situations require T3 supplementation. In some cases, the body has intentionally switched off T3 in order to reduce metabolic activity and protect the cell from inflammation.
It’s still worth measuring TSH, because when it is elevated, then thyroid disease is likely. But a normal TSH does not mean that thyroid disease has been ruled out. In other words:
When TSH is high, the thyroid is under-active. When TSH is low, you just don’t know.
A better way to diagnose thyroid disease
Most thyroid disease is caused by an underlying autoimmune problem. The best test for thyroid disease, therefore, is a test for thyroid autoimmunity called “thyroid antibodies.” Thyroid antibodies correlate with symptoms—regardless of TSH.
Other useful tests: free T3, free T4, reverse T3, SHBG, and thyroid ultrasound.
Symptoms of under-active thyroid (also called hypothyroidism) include hot flushes, fatigue, weight gain, anxiety, depression, heavy periods, high cholesterol, digestive changes, irregular periods, temperature intolerance, brain fog, and memory loss.
Symptoms of overactive thyroid (also called hyperthyroidism) include hot flushes, racing heart, insomnia, and fatigue.
A better way to treat thyroid disease
The best way to treat thyroid disease is to treat the autoimmunity that underlies most cases of thyroid disease. Most of the time, this means removing gluten from the diet and correcting the gut microbiome.
Beyond that, the best way to enhance T3 in the cell is to address the inflammation that suppresses our cellular D1 deiodinase.
For those who do need thyroid hormone supplementation, slow-release T3 or T4/T3 combination is preferable to T4 alone.
T3’s long journey to the thyroid receptor
There is SO much more to the thyroid story.
To reach the cell nucleus where its destiny lies, active thyroid hormone has to undergo a long journey:
- hypothalamus regulation of the pituitary
- TSH release
- TSH action at the thyroid
- T4 production in the thyroid
- carrier proteins in the blood
- conversion of T4 to T3 at the cell
- active transport of T4 and T3 into the cell
- competition of reverse T3 at transport and receptor
- T3 connection with the thyroid receptor
- activation of gene transcription in the cell nucleus.
Inflammation, stress hormones and nutritional deficiencies can play a role at every step along that journey and cause an underactive thyroid.
Fortunately, thyroid disease can be helped by dietary suggestions I discuss in Chapter 11 of my book Period Repair Manual.