The amount of thyroid hormones produced by the thyroid gland is controlled by another hormone, called thyroid-stimulating hormone (TSH). TSH is produced in the pituitary gland. If your thyroid hormones are low, TSH increases in order to stimulate your thyroid to make more hormones. If your thyroid hormones are high, TSH is low, signaling the thyroid to make less hormones. An easy way to remember this is to note that TSH and thyroid hormone levels are usually opposite; if TSH is high, then thyroid hormones are usually low (and vice versa).
High TSH can stimulate the growth of a goiter (an enlargement of the thyroid gland). Therefore, giving extra thyroid hormone in the form of Cytomel will decrease TSH, which can help to shrink goiters.
Cytomel can also be used in diagnostic tests. In people with normally functioning thyroids and pituitary glands, the medication will result in a decrease in uptake of iodine into the thyroid gland (to adjust for a decrease in thyroid hormone production, since the thyroid uses iodine to make thyroid hormones). A special radioactive form of iodine can be given, and its uptake into the thyroid gland can be measured. If the iodine uptake into the thyroid decreases significantly (by 50 percent or more) after giving Cytomel, this indicates that the thyroid can respond normally to signals from the pituitary gland. If it does not decrease, this indicates that the thyroid gland is not responding normally to signals from the pituitary gland.
The thyroid gland makes two different thyroid hormones: levothyroxine (T4) and triiodothyronine (T3). Although T3 is much more active than T4, the thyroid usually produces more T4 than T3. The body can convert the T4 hormone into T3 as necessary. If your thyroid does not make enough thyroid hormones, there are a few different options to increase your levels.
Some forms of thyroid replacement combine T4 and T3 (such as natural thyroid replacement made from pig thyroids). However, such products are usually not the preferred option for most people. Synthetic (manufactured) thyroid hormones like Cytomel are less likely to cause allergic reactions, since they are not made from animal thyroids, and may provide for more precise control of thyroid levels. Although most people take a synthetic T4 thyroid replacement product (such as Synthroid®), some people may benefit from T3 replacement (such as Cytomel), either alone or in combination with a T4 medication. Some people may not convert T4 into T3 appropriately, in which case a T3 product (like Cytomel) would work better.