TRH and hypothyroidism: Hypothyroidism is defined as a deficiency of thyroid hormones and can be further classified as primary, when the dysfunction of the thyroid gland itself causes decreased production of thyroid hormones; secondary, due to inadequate thyroid-stimulating hormone (TSH) production by the pituitary gland; and tertiary, as a consequence of diminished thyrotropin-releasing hormone (TRH) production by the hypothalamus [9].