Hypothyroidism (low thyroid function) is something that those suffering from environmental illnesses often struggle with. Poor thyroid function has been linked to many of then illnesses covered on this site and is often the hidden cause of a host of chronic symptoms including fatigue, poor cognitive function, low mood, low sex drive, cold intolerance, weight gain or inability to lose weight...and many others.

Assuming you have been diagnosed with hypothyroidism (not easy in itself), unless the cause is obvious such as a tumour or autoimmune disease and is detectable using routine tests, chances are you will simply be given a prescription for thyroxine (the main hormone produced by the thyroid gland and also referred to as 'T4') and have your blood levels monitored, along with levels of thyroid stimulating hormone (TSH) - a pituitary hormone that stimulates production of T4 by the thyroid gland.

Unfortunately many patients find that T4 medication doesn't alleviate their symptoms, even when blood levels return to normal, and even if they do feel better they will need to take the medication for the rest of their lives unless the underlying cause of the problem is discovered.

Mechanisms by which Hypothyroidism may Occur

First of all lets look at the main reasons why someone may display symptoms of hypothyroidism:

1. Primary Failure - The thyroid gland itself fails to produce adequate amounts of thyroid hormone e.g. in autoimmune thyroiditis when the immune system produces antibodies that attack and damage thyroid tissue.

2. Control Failure - The systems upstream of the thyroid (the hypothalamus and/or pituitary gland) fail to stimulate and produce enough TSH to trigger thyroid hormone production in the thyroid.

3. Thyroid Hormone Conversion Failure - Although it is T4 that is produced in the largest amounts by the thyroid it is actually another thyroid hormone known as triiodothyronine (T3) that is most biologically active and exerts most of the effects associated with normal thyroid function. Failure of the tissues to convert T4 to T3 can be one cause of hypothyroidism.

4. Thyroid Receptor Resistance - For any hormone to have an effect it must bind to a receptor. Thyroid hormones bind to receptors within the cells (intracellular receptors). These receptors may become insensitive to thyroid hormones due to factors such as genetics, diet, environmental toxins, and chronic illness.

5. Adrenal Fatigue/Exhaustion or Insufficiency - The adrenal and thyroid glands work closely with the thyroid and poorly functioning adrenal glands almost always result in hypothyroidism.

Nutritional, Lifestyle and Environmental Factors that Might Cause these Problems

Nutritional Deficiencies

Any one or a combination of deficiencies of the following nutrients may contribute to hypothyroidism to varying degrees.

- Tyrosine (an amino acid) - Building block of thyroid hormones. - Iodine - Building block of thyroid hormones (e.g. 3 atoms bound to tyrosine in T3, 4 in T4).
- Selenium - Vital part of enzyme that converts T4 to T3, part of glutathione peroxidase which protects the thyroid from free radical damage. - Zinc - T4 synthesis, conversion of T4 to T3, binding of T3 to its receptor.
- Iron - T4 synthesis, conversion of T4 to T3, binding of T3 to its receptor.
- Vitamin E - T4 synthesis, antioxidant (protects the thyroid), aids T4 to T3 conversion.
- Vitamin A - T4 synthesis, needed for formation of T3 receptors and binding of T3 hormone to them.
- Vitamin B2 - T4 synthesis
- Vitamin B3 - T4 synthesis
- Vitamin B6 - T4 synthesis
- Vitamin C - T4 synthesis
- Vitamin D - Regulates the immune system and may protect against auto-immune hypothyroidism. - Omega-3 Fatty Acids - Improve binding of T3 to its receptors.

Foods and Supplements which may Interfere with Thyroid Function

Brassica Family Vegetables - contain chemicals (e.g. isothiocyanates) which inhibit the production of thyroid hormones when consumed in large amounts. The amounts needed to cause significant dysfunction would not usually be obtained from diet alone but may become a problem when supplementing with brassica extracts for other reasons (e.g. as antioxidants, detoxifiers). Brassica vegetables include brocolli, cauliflower, cabbage, Brussels sprouts and kale.

Soy Foods - Soy contains isoflavones which may inhibit the enzyme thyroid peroxidase that is required for T4/T3 production in the thyroid. Isoflavones may also inhibit the response of the cell to T3 once it has bound to its receptor. Soy foods include tofu, miso, and many packaged foods, particularly vegetarian meat substitutes. Isoflavones are marketed as nutritional supplements as an alternative to HRT in menopausal women.

Flavonoids - Antioxidant flavonoids such as quercetin and naringenin found in significant quantities in a variety of fruits may alter thyroid function when consumed chronically in large amounts. Since these flavonoids are marketed as antioxidant nutritional supplements caution must be urged if hypothyroidism is suspected.

L-Carnitine Supplements - L-carnitine is sold as a nutritional supplement to increase energy and stamina since it is required for the production of energy from fats. However, it may inhibit the entry of T4 and T3 into the cells so should be used with caution in hypothyroidism.

Lifestyle Factors

Stress - Cortisol, a hormone produced by the adrenal glands, is required in small amounts for the conversion of T4 to the more potent T3 form. During chronic stress levels of cortisol become chronically elevated which has a negative impact on thyroid function - reducing conversion of T4 to T3 and increasing production of an inactive form of T3 known as 'reverse T3 (rT3)'. In addition high cortisol levels reduce production of TSH by the pituitary. If the chronic stress is severe and/or prolonged then adrenal fatigue/exhaustion may develop and the adrenal glands are no no longer able to produce even normal amounts of cortisol. The conversion of T4 to T3 is also reduced in this situation. Stress therefore can have a major impact on thyroid function and be a significant contributor to hypothyroidism.

Lack of Exercise - Exercise can increase both thyroid hormone production and the sensitivity of cells to these hormones thus improving thyroid function. Lack of exercise may therefore contribute to low thyroid function.

Environmental Factors

Fluoride - Since fluoride is similar in chemical structure to iodine it can compete with iodine and interfere with the iodination of thyroid hormones and slow down their production. Fluoride also competes with TSH and binds to its receptors in the thyroid gland thereby blocking stimulation of the thyroid gland and the production of thyroid hormones. The major sources of fluoride are fluoridated drinking water supplies and toothpaste/dental hygiene products.

Other Environmental Toxins - Other toxins present in the environment can also have a negative impact on thyroid function. Polychlorinated biphenyls (PCBs) for example are structurally similar to thyroid hormones and can bind to thyroid hormone receptors and prevent the hormones themselves from doing they. They may also mimic the action of thyroid hormones depending on the dose involved, causing the opposite of hypothyroidism i.e. hyperthyroidism. PCBs are used in paints, wood floors finishes, adhesives, electrical components and plastic wiring insulation, and a variety of other applications of plastics. 

I could have gone into much more depth on this subject but I hope this post has opened your eyes a little to factors that influence thyroid function and the importance of considering diet, lifestyle and environmental exposures.