In the case of the thyroid, the pituitary gland makes something called “thyroid stimulating hormone,” or TSH for short. TSH stimulates the thyroid gland to make and release several thyroid hormones that regulate your overall level of metabolism. The two most important thyroid hormones are referred to as T3 or T4, because they contain either 3 or 4 iodine atoms.
If the thyroid gland is making too little T3 or T4, the pituitary gland will sense this and will increase the amount of TSH in an effort to stimulate the thyroid gland into making more. If the thyroid is making too much T3 or T4, the pituitary tries to put the brakes on by reducing the amount of TSH it makes.
Although we can measure T3 and T4 directly in the blood, it turns out that thyroid problems can be diagnosed more reliably by measuring TSH (except when there is a pituitary problem, which is much rarer). If the TSH is high, it means the person has hypothyroidism-an under-active thyroid; if the TSH is low, it means the person has hyperthyroidism, or an over-active thyroid.
The only problem is that some people who have very mild thyroid problems have TSH values that are slightly higher than average, but still in the normal range. In this case, if someone comes in with symptoms that suggest the possibility of hypothyroidism, I may give him or her a trial of thyroid medication to see whether it results in improvement. For example, if a woman comes in complaining of fatigue and has a TSH value of 4.1-in the normal range, but higher than I think is optimal-I may give her a trial of thyroid hormone (levothyroxine; brand names Synthroid or Levoxyl). This can be done safely and later stopped if it is not helpful. However, I would only do this after excluding other common causes of fatigue, such as anemia, sleep disturbances, diabetes, depression and the like.