In the 20 years that I have been involved in seeing people with heavy metal toxicity, one of the most frequent and complex issues that has come to my attention is the problem of abnormal thyroid function. Some people show evidence of thyroid dysfunction in their hair test results; almost everyone with heavy metal toxicity will have a low body temperature (97.8º F or below).
 
Why is low body temperature important? Your body's metabolism depends on the activity of thousands of enzymes which act as catalysts for the thousands of biochemical reactions upon which your body's functioning depends. These enzymes are extremely sensitive to temperature.  Lowering the body temperature by even one degree has a profound effect on the rate at which these enzymes catalyze their reactions.  Animal species that cannot maintain their body temperature are forced to hibernate because they cannot function in cold weather.   Low body temperature in humans is almost invariably a sign of impaired thyroid function except for instances of prolonged exposure to low environmental temperatures.

Thyroid hormone made in the thyroid is primarily T4 which is a weak thyroid hormone.  The T4 must be metabolized to T3 in various tissues, since T3 is the most active thyroid hormone (4-5 times more active than T4).  However, this conversion may be impaired for various reasons so that insufficient T3 is available resulting in a hypothyroid state.
 
Toxic metals are a known cause of this problem, particularly where stores of selenium and/or glutathione are depleted.  Such individuals frequently have normal thyroid tests, but are unable to function optimally and are often told by their doctors that their thyroid function is normal.  They may experience a wide variety of problems including cold extremities, lack of energy, muscle cramps, low blood pressure, impaired sleep including sleep apnea, weight gain, constipation, dry hair or hair loss, elevated cholesterol, narcolepsy, impaired balance, asthma, allergies, immune dysfunction with chronic infections, impaired sex drive irregular periods, etc.

Prolonged exposure to mercury and other toxic metals leads to glutathione depletion, affecting metal excretion, immune function, oxidative stress and conversion of T4 to T3.  Mercury toxicity can affect the hypothalamus causing impaired release of TSH.

Persons experiencing severe physical or psychological stress develop impaired thyroid function as well, often because they have inability to detoxify their bodies or because they are producing excessive levels of reverse T3 and T4.
 
 
The question arises:   Does heavy metal toxicity cause thyroid dysfunction  or does thyroid dysfunction lead to heavy metal toxicity?  The answer clearly is: both.  We know that heavy metals and lack of thyroid hormone both cause mitochondrial dysfunction,  impairing energy metabolism.  We also know that impaired energy metabolism inevitably leads to impaired detoxification and impaired excretion of toxic metals and chemicals. Unfortunately, the widespread prevalence of metal toxicity and hypothyroidism is greatly underestimated. Awareness of these problems and their complex inter-relationships  is crucial for optimal health care and optimal health.