A TSH test is a lab test that measures the amount of thyroid stimulating hormone (TSH) in your blood.  The hypothalamus gland monitors the amount of TSH in the blood.  If it believes more or less TSH is needed, instructions are sent to the pituitary gland which produces TSH, and this tells the thyroid gland to make and release thyroid hormones into the blood.
The TSH test is normally stated in milliunits per liter of blood.  A unit is a standard of measurement and a milliunit is one-thousandth of a unit. A liter is a measure of volume that is a little bigger than a quart.  It is expressed as mlU/L.
The signs and symptoms of an overactive thyroid (hyperthyroidism) include the following:

Increase in the heart rate
Feelings of anxiety

Weight loss

Insomnia and sleep difficulties

Feelings of weakness
Problems with the eyes including bulging and dryness and irritation

The signs and symptoms of an underactive thyroid (hypothyroidism) include the following:
Weight gain

Skin dryness

Constipation

Increased sensitivity to cold

Loss of hair

Feelings of tiredness and fatigue

Menstrual problems

NORMAL LEVELS

Many doctors rely on the reference ranges supplied by the labs doing the blood tests.  The labs typically indicate that they believe that the normal TSH reference scale is between 0.5 to 5.0.  However, the American Association of Clinical Endocrinologists states that the correct range is 0.3 to 3.0.  It states, “…In the future, it is likely that the upper limit … will be reduced to 2.5 mIU/L because >95% of rigorously screened normal euthyroid (normal thyroid) volunteers have serum TSH values between 0.4 and 2.5 mIU/L….”

HIGHER THAN NORMAL LEVELS OF TSH

Higher than normal TSH levels are most often due to an underactive thyroid gland (hypothyroidism) which can lead to problems including the following:

Hashimoto’s thyroiditis (an autoimmune disorder)
Diabetes
Infertility
Menstrual problems

Depression

LOWER THAN NORMAL LEVELS OF TSH

Lower than normal TSH levels are most often due to an overactive thyroid gland (hyperthyroidism) which can lead to problems including the following:
Graves’ disease (enlargement of the thyroid)

Toxic nodular goiter (swelling of the thyroid)