Thyroid dysfunction is abnormal functioning of the thyroid gland. This can be portrayed as either an underactive (hypothyroid) or overactive (hyperthyroid) thyroid.
Thyroid Hormone Production in the HPA-T Axis
Thyroid hormone production begins with the hypothalamus, which releases thyrotropin-releasing hormone (TRH) to the pituitary gland. The pituitary gland then releases thyrotropin-stimulating hormone (TSH) to the thyroid gland. The thyroid gland then releases T4 and T3. Inactive T4 is converted to active T3, the thyroid hormone that controls metabolism.
Metabolic Syndrome and Other Thyroid Disorders
Metabolic Syndrome (MetS) can be described as the conglomeration of various conditions including insulin resistance, irregular blood glucose levels, excess body fat, abnormal cholesterol or triglyceride levels in the bloodstream, and dysfunction of the inner lining of the blood vessels. These factors increase the risk of heart disease, diabetes, and stroke. If at least three of the following risk factors are present in an individual, MetS may be diagnosed: 
- Abdominal obesity (“apple shape” body type)
- High blood pressure
- High triglyceride level
- Low HDL cholesterol level
- High fasting glucose
Those with metabolic syndrome may be at higher risk for thyroid disorders, and vice versa. Most research has found links between decreased thyroid activity and metabolic syndrome.
TSH and Metabolic Syndrome
One study that analyzed over 7,000 euthyroid subjects found that increasing TSH concentrations significantly increased the number of components of MetS.
Hypothyroidism and Obesity
Overt hypothyroidism (hypothyroidism with clear symptoms) is associated with weight gain or inability to lose weight. A cross-sectional study found that there is a positive association between obesity and subclinical hypothyroidism (hypothyroidism with mild symptoms). 
Insulin Resistance and Glucose Transport
Insulin resistance has also been linked to thyroid dysfunction. Studies have revealed that those with subclinical and overt hypothyroidism have higher baseline fasting glucose levels and decreased glucose transport.  Though less clear in subclinical hypothyroid individuals, increased triglycerides were found in approximately 1/3 of all overt hypothyroidism patients. 
So, we see that MetS is associated with low thyroid function, which contributes to weight gain and therefore further insulin resistance. It is a vicious circle for which dietary, nutritional and even medical intervention is necessary if the circle is to be interrupted.
How To Improve Thyroid Function
Ways to improve thyroid function include:
- Optimize iodine sufficiency (except in Hashimoto’s thyroiditis)
- Optimize cofactor sufficiency: selenium, iron, vitamin A, zinc, copper
- Avoid heavy use of goitrogens, such as soy and raw cruciferous veggies
- Increase exercise
- Correct hypoglycemia (not enough blood sugar to promote T4 conversion)
 National Heart, Lung, and Blood Institute. 2016. What is Metabolic Syndrome? Retrieved Aug 2, 2016 from http://www.nhlbi.nih.gov/health/health-topics/topics/ms
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