Thyroid hormones stimulate energy production, so deficiency of them causes generalized tiredness and lethargy. There may be also muscle weakness, cramps and slow heart rate, dry skin and flaky ski, hair loss etc. In addition the skin and other body tissues may thicken and there may be weight gain, a syndrome known ask myxedema. In some cases, a goiter(enlargement of thyroid gland) develop, although not all goiters are due to hypothyroidism.

Normally, the thyroid gland pumps enough thyroid hormone into the blood to cover all of the body’s needs. Thyroid hormones include T4 (tetraiodothyronine, thyroxine) and T3 (triiodothyronine). T4 and T3 circulate in the blood primarily bound or linked to protein molecules. Thyroid carrier proteins include thyroxine binding globulin (TBG), albumin, or transthyretin (TTR). Linked to these proteins, thyroid hormone isn't available to the body’s cells. Measurement of this protein bound thyroid hormone is referred to as a "total" level. Total T4 and to a lesser extent total T3 levels are affected by the concentrations of protein in the blood. Certain medications, hormones such as estrogen, other non-thyroidal illnesses and liver problems can cause alterations in protein concentration. Influenced by protein alterations, the total T4 and T3 measurements may not accurately represent thyroid function.