There are two types of thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T4 is inactive and kept in reserve, T3 is the active. Thyroid hormones control the growth, differentiation and metabolism of each cell in the body. They also control how fast our body uses the fuel that we consume, particularly carbohydrates and fat. This, in turn, helps to regulate our temperature and body fat percentage. The thyroid hormones T3 and T4 are produced in the thyroid gland. This gland uses iodine to produce various hormones. We get the needed iodine from the foods we consume. About 80% of the thyroid hormones we produce is T4, the inactive thyroid hormone that is typically held in reserve by the body. T3 makes up only 20% of the thyroid hormone, but is about four times “stronger” than T4, and is the active hormone that the body uses to function. T4 is converted into T3 when thyroid hormone is needed.
The release of the thyroid hormones is controlled by the thyroid stimulating hormone (TSH), which is produced in the pituitary gland. Low circulating levels of thyroid hormone are detected by the hypothalamus which then instructs the pituitary to release TSH. When sufficient amount are released the hypothalamus communicates with the pituitary to stop or slow down. Because of this complicated feedback loop high levels of TSH in the blood often means the pituitary is trying to stimulate production of the thyroid hormone but the thyroid gland is not responding. This condition known as hypothyroidism is more common in women and the elderly. However, a secondary type of insufficiency is ineffective conversion of T4 to T3 as T4 is converted to the inactive reverse T3 instead.
Since thyroid hormones are bound to proteins while circulating in our bodies, it is the unbound, available, “free” hormones that should be measured. Physicians frequently order only a TSH or a free T4 test, but the most effective testing will measure for free T3, free T4 and TSH levels. Thyroid hormone levels in the low or normal range, together with a high TSH confirm a thyroid inadequacy or possibly a low T3 syndrome. With low T3 syndrome, T4 cannot be converted in T3, resulting in normal T4 levels, but low T3 levels.
Naturally derived thyroid, with its mixture of T4 and T3 is the ideal replacement because it mimics the natural thyroid hormones produced by the body. Unfortunately, this is no longer the treatment most patients are offered. It has now become routine practice to prescribe synthetic T4 alone. This mass-produced and inexpensive T4 may not be converted to the T3 the body needs. Appropriate treatment, as indicated by our studies and that of others, involves administration of T4 and T3, while monitoring Free T3 levels.