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Two reviews have highlighted the limitations of standard thyroid testing. Most conventional practitioners test TSH (thyroid stimulating hormone) levels as a first port of call when assessing thyroid function, however these recent reviews have shown it to be an unreliable marker of hypothyroidism that may be missing many cases.

Both reviews were published in the Journal of Restorative Medicine and found TSH testing to be unreliable in certain cases of hypothyroid. The authors advised that extreme caution should be used when TSH levels are shown to be normal yet patients exhibit typical signs of hypothyroid, particularly alongside conditions such as physiologic and emotional stress, depression, dieting, obesity, leptin resistance, diabetes, chronic fatigue syndrome, fibromyalgia, inflammation, autoimmune disease, or systemic illness.

The authors call for healthcare practitioners to put a higher emphasis on symptom evaluation and for more comprehensive testing when assessing whether hypothyroidism may be at the root cause of patients’ symptoms.

Thyroid health is complicated – so we’ve put together a simplified summary of this latest research:

How thyroid hormones are produced
• The pituitary gland is often called the master gland as it controls most hormone production and regulation throughout the body.

• The pituitary gland produces Thyroid Stimulating Hormone (TSH) when it detects low intracellular levels of tri-iodothyronine (T3) in the pituitary.

• TSH then stimulates the thyroid gland to produce Thyroxine (T4) and small amounts of Tri-iodothyronine (T3).

Once in the blood, the conversion of the thyroid hormones into their active forms happens at a cellular level in the target tissues. T4 is converted into T3 (active form) and reverse T3 (inactive form). This is known as peripheral conversion. Thyroid hormones stimulate cellular activity in relation to energy production.

• The local control of cellular thyroid levels is mediated through three different selenium-dependent deiodinase enzymes present in different tissues in the body.

• The activity of deiodinase enzymes changes in response to differing physiologic conditions. This local control of intracellular T4 and T3 levels results in different tissue levels of T4 and T3 under different conditions.

• Pituitary levels are under completely different physiologic control and T3 levels will always be significantly higher in the pituitary than anywhere else in the body – making TSH unreliable under numerous circumstances.

What happens when it goes wrong?
• Reduced T4 and T3 transport into the cells in peripheral tissues is seen with a wide range of common conditions, including insulin resistance, diabetes, PMS, depression, bipolar disorder, hyperlipidemia, chronic fatigue syndrome, obesity, leptin resistance, fibromyalgia, inflammation, autoimmune conditions, neurodegenerative diseases, chronic illness, migraines, physiologic and emotional stress, anxiety, infection / fever, systemic illness, chronic dieting, low protein diet, carbohydrate withdrawal, chronic pain, ageing and exposure to toxins and plastics. Intracellular T3 level in the pituitary however often remains unaffected, despite low T3 in peripheral tissues.

• Thus the clinical picture in these cases may be normal levels of T3 in the pituitary but low levels in peripheral tissues. TSH levels will present as normal (the pituitary is not sensing the low thyroid hormone levels in peripheral tissues) but patients will experience significant hypothyroid symptoms.

• The contrast between thyroid hormone levels in pituitary and peripheral tissues that can occur in many common and chronic conditions is what results in many cases of undiagnosed hypothyroid when practitioners rely solely on TSH testing. 

The authors concluded;

If a patient with a normal TSH presents with signs or symptoms consistent with hypothyroidism, which may include low basal body temperature, fatigue, weight gain, depression, cold extremities, muscle aches, headaches, decreased libido, weakness, cold intolerance, water retention, slow reflex relaxation phase or PMS, a combination of both clinical and laboratory assessment should be used to determine the likely overall thyroid status.”

It is essential to take into account the effect that many common and chronic conditions can have on thyroid function and how this alteration in function may not show on standard thyroid tests. For more thorough evaluation of thyroid health, testing must include a more comprehensive range of markers in conjunction with clinical assessment. 

Holtorf K. Thyroid Hormone Transport into Cellular Tissue.  Journal of Restorative Medicine 2014; 3: page 53-68
Holtorf K. Peripheral Thyroid Hormone Conversion and Its Impact on TSH and Metabolic Activity.  Journal of Restorative Medicine 2014; 3:  page 30-52

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