Thyroid Function Tests
Thyroid function tests are done to estimate the amount of thyroid hormones circulating in the blood. These tests reflect the normal, over, or under functioning of the thyroid gland. A normally functioning thyroid is said to be euthyroid. When a person has an overfunctioning thyroid gland, he is said to be suffering from hyperthyroidism, whereas if the thyroid gland is underperforming, the condition is known as hypothyroidism.
These blood tests are usually advised if a patient experiences excessive fatigue, is intolerant to cold, has dry and flaky skin, and suffers from constipation, all of which are common signs and symptoms of hypothyroidism. On the other hand, if an individual complains of profuse sweating, tremors, velvety skin, palpitations, and diarrhea, he could be a case of hyperthyroidism. Enlargement of the thyroid gland is medically called goiter. This may or may not be accompanied by alterations in the levels of the thyroid hormones.
A thyroid function test usually involves the measurement of three hormones, namely TSH (Thyroid Stimulating Hormone), T3 (Triiodothyronine), and T4 (Thyroxine). An increase in the TSH level (the normal range is 0.4–4.2 mU/L) in the blood is indicative of hypothyroidism. If the level drops below normal, this is considered to be a sign of hyperthyroidism. T4 is a hormone that is secreted by the thyroid gland and is largely responsible for stimulating all the metabolic activities of the body.
The total T4 level (the normal range is 5–12 μg/dL or 65–155 nmol/L) is known to increase during pregnancy and other high estrogen states, including the use of birth control pills or estrogen replacement therapy. Typically, the level of T4 increases in hyperthyroidism and decreases in hypothyroidism. The free T4 test measures the amount of unbound thyroxine in the blood. Its level is indicative of the amount of the hormone that is readily available for uptake and use by the cells of the body. Its level normally ranges between 0.58 and 1.64 ng/dL or 7.48 and 18.06 pmol/L.
T3 is an active hormone that gets produced when thyroxine gets metabolized in the peripheral tissues. Its level in the blood normally ranges between 80 and 230 ng/dL or 1.2 and 3.5 nmol/L. Free T3 (normal values range between 0.2 and 0.5 ng/dL or 3.5 and 7.7 pmol/L) refers to the amount of unbound triiodothyronine in the bloodstream. The free T3 test is considered to be more accurate in the diagnosis of thyroid disorders as compared to the total T3 test. T3 levels are known to increase in hyperthyroidism and decrease in hypothyroidism.
The T3 resin uptake (T3RU) test, also referred to as the T7 test, measures the amount of sites available to bind with the thyroid hormones for transport. Elevated T3RU is a common feature of hyperthyroidism. Thyroglobulin (TG) is an iodine containing protein that is barely found in a normal thyroid gland. Its level increases considerably in conditions such as Graves' disease, thyroiditis, or thyroid cancer. Under stress, our body conserves energy by producing Reverse T3 (RT3), an inactive form of the T3 hormone, instead of the usual conversion of T4 into T3, the biologically active form of the thyroid hormone. The level of RT3 can also be estimated in the laboratory.
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