Hormone Series Part 2: Thyroid Hormone.

In the first part of our hormone series, we discussed the energy regulating fuel gauge of the body leptin. Leptin has a profound effect on many parts of metabolism, but one of its most pertinent effects is to affect the amount of thyroid hormones the body produces. Leptin interacts with pathways that influence thyroid hormone production, more specifically thyroid stimulating hormone (TSH), in the central processing and hormonal regulating regions of the brain. When energy stores become low, especially for a consistent period of time such as during a fat loss diet, then the brain detects falling levels of leptin and this is then reflected by a reduction in TSH entering the blood stream. This lowering of TSH means that the thyroid gland (a butterfly shaped gland surrounding the windpipe) reduces its output of thyroid hormones and this can have an effect on our metabolic rate, and is one of the reasons why our metabolic rate starts to slow when we lose body fat.


Thyroid hormones released from the thyroid gland include thyroxine (T4) which can be considered a ‘prohormone’ for the body’s most active form of thyroid hormone, triiodothyronine (T3), which is converted to its active T3 form by specialised enzymes in the cells of the body. T4 is produced in much higher amounts than T3 (around 15:1) but T3 is much more active, being around three to four times more effective. Thyroid hormones are manufactured in the body from iodine and the amino acid tyrosine, with selenium also playing an essential part in the generation of T3. Aside from regulating energy production, thyroid hormones also play a key role in bone development and remodelling and growth regulating actions with growth hormone as well as vitamin and mineral metabolism. The more active form, T3, can have a profound effect on heart rate, breathing rate and nervous system activity.


Those with hypothyroidism have low levels of circulating thyroid hormones and tend to gain weight easily, as there is a marked reduction in metabolic rate. From a medical perspective, low levels of T3 in the body can be caused by interruptions to TSH production, a lack of output of T4 from the thyroid gland or an inability to sufficiently convert T4 into T3. For those with an overactive thyroid (hyperthyroidism) they may struggle to gain weight and have a leaner look as their basal metabolic rate is much higher than in normal people. Most people who are on a fat loss diet would be unlikely to push themselves significantly outside of the normal ranges of thyroid production; however severe dieting stress can have a profound effect on hormone levels and metabolic rate.


Thyroid hormone influences energy output by regulating the rate of fuel uptake and usage in the body’s power plants, the mitochondria. This means thyroid hormone has an intrinsic link with carbohydrate, fatty acid and protein metabolism/catabolism. Thyroid hormone acts directly within the nucleus of the cells, binding to nuclear receptors that then signal specific parts of the genetic code within our DNA to produce proteins. These proteins are the enzymes (and other functional components) that allow fuel usage to take place at a quicker rate when under increased cellular signalling in the presence of higher levels of thyroid hormone. On the other side of the coin, a reduced rate in energy output and fuel usage is evident when thyroid levels are low and the specific parts of our protein producing parts of our genetic code are not generating these enzymes at normal levels.


So are thyroid levels worth worrying about? For most of us thyroid levels are unlikely to be something which we need to worry about, as long as we are eating enough of the right foods. Tyrosine is found in dairy, meat, soy, grains, nuts and seeds. Iodine is found in meat, iodized salt, dairy and seaweed/kelp and selenium are present in meats, seafood and Brazil nuts. Even during a fat loss phase, reductions in thyroid hormone are unlikely to be profound enough to worry about or in any case be controlled. If you do struggle to lose weight even on very low calories it might be worth while taking a trip to the doctors just to get your thyroid levels checked out for peace of mind that everything is working properly.