The thyroid (or thyroid) is a butterfly-shaped gland located at the base of the neck, below the larynx. The thyroid produces two hormones: triiodothyronine (T3) and thyroxine (T4), which act to regulate our metabolism, that is, the way the body uses and stores energy.
Hypothyroidism – Thyroid Function
Hypothyroidism is the name that occurs when the thyroid gland produces an insufficient amount of these hormones. To understand how hypothyroidism arises it is necessary to know how the thyroid works. The mechanism explained below looks complicated, but it is not. Read it calmly that everything is well explained.
1. As I just explained, the thyroid produces two hormones that control our metabolism, called T3 and T4.
2. The pituitary gland, located at the base of the brain, controls the degree of T3 and T4 production by the thyroid through a hormone called TSH (Thyroid Stimulating Hormone).
When there is little circulating thyroid hormone, the pituitary gland increases the secretion of TSH, giving order for a greater production of T3 and T4 by the thyroid. When there is too much-circulating hormone, the pituitary gland decreases TSH secretion, discouraging the thyroid to produce T3 and T4. Thus, the organism can maintain always stable levels of T3 and T4, keeping our metabolism controlled.
Primary hypothyroidism vs. secondary hypothyroidism
Hypothyroidism can be caused by a thyroid problem that prevents the secretion of your hormones (primary hypothyroidism) or a problem in the pituitary gland that inhibits TSH secretion, thereby inhibiting the production of T3 and T4 (secondary hypothyroidism).
Therefore, primary hypothyroidism occurs by thyroid defect and secondary hypothyroidism by the defect in the pituitary gland.
The distinction between primary and secondary hypothyroidism is very easy, being made through the measurement of TSH and T4 in the blood:
When the thyroid has a problem and begins to produce a little hormone, the pituitary progressively increases its production of TSH to try to get around this deficit. Therefore, in primary hypothyroidism, we find elevated TSH and low T4. If the problem is central, in the pituitary, we will find low TSH due to lack of secretion and a T4 also low because of lack of stimulation for its production.
95% of cases of hypothyroidism are of primary origin. The main causes are Hashimoto’s thyroiditis, surgical removal of the thyroid (thyroidectomy) and destruction of the gland by irradiation.
Thyroiditis of Hashimoto
Hashimoto thyroiditis is an autoimmune disease (read: AUTOIMMUNE DISEASE), which occurs by a destruction of the gland by our antibodies. Hashimoto is the main cause of hypothyroidism, being seven times more common in women than in men.
Hashimoto’s thyroiditis occurs as follows:
For reasons unknown, our body starts to produce antibodies against the thyroid gland itself. The process of destruction is slow and lasts for several years.
As the thyroid cells are being destroyed, the ability of the gland to produce T4 and T3 goes down. Noting that there is a drop in thyroid hormone levels, the pituitary gland increases the secretion of TSH by stimulating thyroid cells that still exist to increase their production of hormones.
This increase in TSH is sufficient to normalize T3 and T4 levels. Therefore, in the early stages of Hashimoto’s thyroiditis, the patient does not present symptoms, since their blood levels of T3 and T4 remain normal. However, in the blood tests, we have already been able to detect a TSH that is higher than normal. This phase is called subclinical hypothyroidism (read: SUBCLINICAL HYPOTHYROIDISM).
As more cells die, more TSH is secreted by the pituitary, to the point that the remaining cells are so few that they can no longer produce the T3 and T4 needed to maintain the desired blood level. When thyroid hormones become low, the symptoms of hypothyroidism begin to appear.
The name thyroiditis is given because the action of the antibodies in the thyroid causes an irritation of the same. It is possible that the patient develops hyperthyroidism in the early stages since the irritated gland can begin to release more hormones than desired. The patient may evolve initially with hyperthyroidism, and only in later stages can have hypothyroidism.
Symptoms of hypothyroidism
Increased thyroid volume, called goiter.Weakness, discouragement and easy fatigue.
The decrease in sweating.
Light weight gain (hypothyroidism does not lead to large weight gains).
Pain in the joints.
Carpal Tunnel Syndrome
Changes in menstruation (more or less).
Loss of libido.
Reduction of eyebrow hairs.
Swelling (in more severe cases).
Eat (in severe and untreated cases).
Hypothyroidism leads to weight gain but is NOT CAUSED by OBESITY. It is very common for people to justify their obesity by hypothyroidism, when in fact, this condition leads to the gain of only a few pounds, at most 5 or 6. Also, weight gain is largely by fluid retention and not by gain of fat. Nobody gains enough weight to become obese just because they have hypothyroidism. Obesity only occurs in those patients who were previously overweight, already close to being obese.
In children, hypothyroidism leads to a low growth and mental retardation called cretinism (hence the origin of the word cretin). Thyroid hormones are essential for brain development. The foot test is used to diagnose hypothyroidism in newborns.
Hypothyroidism caused by removal or irradiation of the thyroid exhibits the same symptoms of hypothyroidism caused by Hashimoto’s thyroiditis. The difference is that its onset is abrupt in thyroid removal and slow and progressive in Hashimoto.
Another cause of hypothyroidism is the lack of iodine, a substance required for the production of hormones. Today there is iodine supplementation in cooking salt, and this type of hypothyroidism is rare in urban areas.
Some medications, such as amiodarone, interferon and lithium can also cause changes in thyroid function, causing hypothyroidism.
For more details on the symptoms of hypothyroidism, read SYMPTOMS OF HYPOTHYROIDISM.
Diagnosis of hypothyroidism
The diagnosis is made with TSH, free T4 and thyroid antibodies (anti-TPO and anti-thyroglobulin).
In general, the diagnosis of hypothyroidism is given to patients with symptoms of hypothyroidism who have TSH greater than 4 mU / L. There is also the group that falls under the definition of subclinical hypothyroidism, that is, TSH greater than 4 mU / L, but without symptoms of the disease. In the latter case, treatment is only necessary if the patient has high cholesterol, TSH greater than 10 mU / L, if the patient is pregnant, or if the thyroid antibodies are positive (anti-TPO and anti-thyroglobulin).
The presence of anti-TPO or anti-thyroglobulin in a patient with hypothyroidism indicates that the cause is Hashimoto’s disease (read: ANTIBODIES AGAINST TIREOID: anti-TPO, TRAb, and anti-thyroglobulin).
Today, thanks to the identification of subclinical hypothyroidism, we are already able to diagnose the disease before it presents clinical signs. One of the first changes is the elevation of cholesterol, which may precede in years the onset of frank hypothyroidism.
Attention, there is no hypothyroidism with normal analyzes. If you think you have symptoms of hypothyroidism, but your TSH and T4 are normal, your complaints have another cause.
All patients older than 50 years should have their TSH dosed, even if they do not present any symptoms of hypothyroidism. If there is a positive family history, screening can begin at age 35, especially in women.
Treatment of hypothyroidism
There is no cure for Hashimoto’s disease, but fortunately, there are already synthetic thyroid hormones. The treatment of hypothyroidism consists of the simple daily administration of these. The drug normally used is Levothyroxine (Puran®, Synthroid®, Letter®), which is a synthetic T4 (read: LEVOTIROXIN (Puran T4) – Indications, Doses and Side Effects).
Levothyroxine is given once a day and should always be taken on an empty stomach (1 hour before or 2 hours after eating). The goal of treatment is to keep TSH within the normal range, ranging from 0.4 to 4.0 mU / L. For this your doctor may have to change the doses of the medicine from time to time.
The dosages of levothyroxine tablets are in micrograms and not in milligrams, as are most drugs. Therefore, levothyroxine should not be made in a pharmacy for handling, so there are no errors in dosing.
The symptoms usually regress after two weeks of treatment. The goal is to keep the patient with normal and symptom-free TSH.
The treatment is lifelong and can not be discontinued.
Treatment of subclinical hypothyroidism
There is no doubt that patients with elevated TSH and symptoms of hypothyroidism should be treated with hormone replacement. The doubt arises in cases of subclinical hypothyroidism.
In this group, treatment is usually only necessary if the patient has high cholesterol, TSH greater than 10 mU / L, if the patient is pregnant, or if the thyroid antibodies are positive (anti-TPO and anti-thyroglobulin).
Author Dr. Pedro Pinheiro May 2, 2017