Hypothyroidism and TSH Levels

The thyroid gland is located in the middle line of the neck, surrounding the cricoid and tracheal cartilages, below to what it is called Adam’s apple. The thyroid gland is the only gland in the body that secretes the thyroid hormones, triiodothyronine (T3) and thyroxine (T4), and also produces the hormone calcitonin. The process of T3 and T4 production occurs when iodine is attached to the tyrosine (an amino acid) contained in a protein named Thyroglobulin, and because a group of biochemical reaction and enzymes, the thyroid hormones are released into the blood.

T4 (thyroxine) is secreted in a higher amount, and this one is converted into T3 (triiodothyronine). The T3 is bonded by a thyroid hormone receptor inside the cells, where stimulates the protein synthesis. Also, it binds an integrin in the cell membrane to stimulate the blood vessel production and the cell growth. These hormones increase the strength of the heartbeat, the consumption of oxygen and the activity in the mitochondria; as a result, the body’s temperature and the blood flow increase too. Thyroid hormones have a metabolic effect: increasing the basal metabolic rate, the appetite, the gut motility, the absorption and breakdown of glucose, and stimulating the breakdown of fats.

In the hypothalamus, the thyrotropin-releasing hormone (TRH) is produced and stimulates the anterior pituitary gland to secretes the thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce thyroid hormones; the produced hormones give negative feedback to the pituitary and the hypothalamus, decreasing the production of TSH and TRH. Lower TSH may lead to lower TSH and then to reduced thyroid hormone production. This hypothalamic-pituitary-thyroid axis keeps the hormone levels within reasonable limits.

The TSH controls the use and incorporation of the iodine in the molecules of thyroglobulin. Because iodine is necessary to thyroid hormone production, lower levels of Iodine in the bloodstream -because a reduced daily intake or absorption- can result in a decrease of thyroid hormones production.

Pathology o abnormal status of the thyroid gland, like inflammation, cancer, nodules, swelling, and lower hormones production and regulation, can be developed as a result of iodine deficiency levels, congenital disease, medications, thyroid surgery, radiation therapy, pituitary disorder, and autoimmune disease. Medicines that can affect the thyroid functioning are amiodarone, interferon alfa, lithium-based mood stabilizers, and tyrosine kinase inhibitors.

In the presence of thyroid hormones levels alterations, symptoms will appear according to the excess or lack of them. High levels of thyroid hormones are known as hyperthyroidism, and the symptoms of this disease include anxiety, tremors in hands, bulging of the eyes, weight loss, palpitations, nervousness, insomnia, intolerance to the heat, and in some cases hair loss and muscle weakness.

When thyroid hormones are lower (hypoactive thyroid), it is called hypothyroidism. Symptoms of hypothyroidism are tiredness or fatigue, slow heart rate, weight gain, constipation, hair loss, dry skin, muscle weakness, muscle aches, joint pain, hoarseness, heavy menstrual bleeding, cold intolerance, high cholesterol level, infertility, depression, and impaired memory. Untreated hypothyroidism, over the time, can lead to Goiter (enlarged thyroid) or myxedema, a life-threatening condition.

During pregnancy, the thyroid gland increases its size and thyroxine production increases also, using more of the iodine available in the bloodstream; as a result, many women can develop hypothyroidism during the pregnancy or after giving birth.

Diagnosis

Screening for hypothyroidism or thyroid disease can be done when patients present symptoms related to the gland-related illness such as tiredness, dry skin, constipation, weight gain, increased heart rate. If the person had thyroid problems previously, or is an older woman, or is an expecting mother, it is recommended to get screened too.

The laboratory tests used to determine the function or pathology of the gland will be the TSH, T3 and the Thyroxine hormone (t4). TSH measurement is a highly sensitive test that can be affected or show abnormal levels, even when symptoms do not appear yet. Because of its sensitivity, this is the primary test used to determine not only hypothyroidism, but its measurement is the way to obtain information about the response of the body to the treatment and will allow determining the dosage of medication to be used in each patient at the beginning of the treatment and over time.

If T4 is low and the TSH is elevated, we are in the presence of an underactive thyroid. It is possible when the pituitary gland produces more TSH to stimulate the thyroid gland to produce more thyroid hormone. In primary hypothyroidism, THS is elevated, with low T4 and high TRH. In Secondary hypothyroidism, TSH is decreased, with low t4 and elevated TRH. In subclinical hypothyroidism, signs or symptoms are not always present, and it is only because of the higher TSH that this condition can be diagnosed when T3 and T4 are normal.

TSH role in the treatment of Hypothyroidism

After lab test are performed, and the diagnosis of hypothyroidism is made, the Levothyroxine -a synthetic form of T4 considered the first choice of treatment- or the natural thyroid hormones obtained from the pig or cow, will be prescribed as a treatment for this condition. When the person taking the hormone has three months of use, new TSH levels are needed to determine if the dosage used is the correct for him. If TSH levels still show clinical evidence of hypothyroidism, a dose adjustment it is necessary. Every three months after a new medication dose adjustment, TSH levels need to be obtained, and after the finding of lab results is within the expected limits during two or three of these exams, the TSH level will be tested each year over time as a routine, or in the presence of thyroid disease symptoms.

The thyroid-stimulating hormone TSH plays a fundamental role in the formation of the thyroid hormones, but also, is the principal indicator -measured with laboratory exams- in the diagnosis of the hypothyroidism and the selection of the right amount of medication to be given to a patient to reverse this disease.

References:

1. Guyton, A. and Hall, J. (2010). Textbook of medical physiology. Philadelphia: Elsevier Saunders.