In this condition, the typical symptoms of overt hypothyroidism are scarce or absent: the increase in TSH levels manages to keep the thyroid hormone values in the normal range.
The most frequent cause of subclinical hypothyroidism is Hashimoto's thyroiditis.
Thyroid: key points
Before defining the characteristics of subclinical hypothyroidism, it is necessary to briefly recall some notions relating to the thyroid gland:
- The thyroid is a small endocrine gland, located in the anterior region of the neck, in front of and lateral to the larynx and trachea. The main hormones it produces - thyroxine (T4) and triiodothyronine (T3) - control metabolic activities and are responsible for the proper functioning of most of the body's cells.
- More specifically, the thyroid hormones indicate how fast the body must work and how it must use food and chemical substances to produce energy and perform its functions correctly. Not only that: the thyroid intervenes in the growth and development processes of many tissues. and stimulates cellular activities, optimizing, in particular, the functions of the cardiovascular system and the nervous system.
- Thyroid hormone production is activated and deactivated via a feedback system. Among the various factors involved in this mechanism, the thyroid stimulating hormone (TSH) is responsible for maintaining the concentration of thyroid hormones stable in the bloodstream.
I am an example:
- Hashimoto's thyroiditis (leading cause of subclinical hypothyroidism);
- Basedow-Graves disease.
Other causes of subclinical hypothyroidism can be:
- Previous acute inflammation;
- Iodine deficiency (dietary: diet poor in iodine or rich in foods, called "gozzigeni", which hinder its assimilation; endemic: long stay in iod-deficient geographical areas, especially mountainous and far from the sea);
- Iatrogenic, in particular:
- Previous ablative therapy with radioactive iodine;
- Thyroid removal surgery (thyroidectomy);
- Drugs (amiodarone, lithium, iodine-containing radiological contrast media, etc.);
- Insufficient replacement therapy;
- External radiotherapy of the head and neck (administered, for example, in case of laryngeal carcinoma, Hodgkin's lymphoma, leukemia, intracranial neoplasms, etc.).
Subclinical hypothyroidism can also present itself in an idiopathic form (ie for unidentifiable causes).
Who is most at risk
Subclinical hypothyroidism is relatively common (prevalence is estimated to be between 4 and 10% in the general population).
The condition affects mainly with advancing age and in the female sex ("critical" periods for thyroid function are pregnancy and menopause).
Subclinical hypothyroidism is particularly common in those with underlying Hashimoto's thyroiditis.
The subjects most predisposed to developing subclinical hypothyroidism are:
- Down syndrome patients;
- Women in the postpartum period (within 6 months);
- Menopausal women;
- Elderly patients;
- Patients with type 1 diabetes mellitus;
- Patients with heart failure;
- Patients with a family history of thyroid disease;
- Patients with other autoimmune diseases.
It should be remembered that subclinical hypothyroidism is a condition in which the alteration of thyroid function is mild to moderate. If it is neglected, however, the dysfunction can progress to full-blown hypothyroidism (circulating TSH levels are elevated and thyroid hormone values are below normal limits, so they are insufficient to maintain a state of euthyroidism).
Subclinical hypothyroidism: main symptoms
Manifestations of subclinical hypothyroidism can be subtle or mild.
Symptoms usually occur after a long subclinical course and may include:
- Muscle weakness
- Asthenia;
- Daytime sleepiness;
- Cold intolerance;
- Difficulty concentrating
- Hoarseness;
- Dry and rough skin;
- Eyelid edema;
- Loss of memory
- Constipation.
In most cases, subclinical hypothyroidism remains stable for several years and can sometimes regress.
The risk of subclinical hypothyroidism of progressing towards the overt form is greater in elderly patients and in those with high anti-thyroid antibody values (a parameter indicative of the presence of autoimmune diseases).
Problems associated with subclinical hypothyroidism
In recent years, several scientific studies have associated subclinical hypothyroidism with various clinical conditions.
In addition to the possible progression of the dysfunction up to full-blown hypothyroidism, there may be:
- Increase in the level of low-density lipoproteins;
- Increased cardiovascular risk;
- Cognitive decline (in older patients);
- Anxiety and depression.
Additionally, patients with subclinical hypothyroidism are more likely to develop:
- Hypercholesterolemia (increase in the level of total cholesterol);
- Atherosclerosis;
- Dyslipidemia;
- Coronary artery disease;
- Peripheral arterial disease.
The diagnosis of subclinical hypothyroidism can be made based on:
- Careful history of the patient;
- Presence of symptoms and signs of mild thyroid gland hypofunction;
- Measurement of serum concentrations of TSH, free T4 (FT4) and free T3 (FT3) after a simple blood sample.
Subclinical hypothyroidism is characterized by elevated serum levels of TSH (thyroid stimulating hormone) associated with normal levels of free thyroid hormones (FT3 and FT4) on two occasions at least 2-3 months apart.
The detection of anti-Thyroglobulin antibodies (Ab anti-TG) and anti-Thyroperoxidase antibodies (Ab anti-TPO) in the blood allows to establish the autoimmune etiology of subclinical hypothyroidism and the opportunity to start replacement therapy with L-Thyroxine ( L-T4).
Thyroid ultrasound, scintigraphy and fine needle aspiration are a useful completion for the evaluation of the clinical case, as they provide information on the morphology and functional capacity of the thyroid.
What tests are needed for subclinical hypothyroidism?
The blood tests useful for the diagnosis of subclinical hypothyroidism are:
- Dosage of TSH, FT3 and FT4 (free form of T4);
- Stimulus test with TRH (thyrotropin releasing hormone);
- Dosage of anti-thyroperoxidase antibodies (Ab anti-TPO) and anti-thyroglobulin (Ab anti-TG);
- Total cholesterol, HDL, LDL and triglycerides dosage.
In subclinical hypothyroidism, circulating thyroid hormone levels are typically found within normal limits, associated with an elevated serum TSH value. The dosage of anti-thyroid antibodies allows us to indicate the presence of antibodies responsible for the most common form of hypothyroidism. that is, the autoimmune one.
What to do when high TSH is encountered?
The first thing to do is to repeat the TSH dosage, after 2 or 12 weeks to exclude a "transient anomaly. The evaluation of the" FT4 is useful for defining the condition of subclinical hypothyroidism and allows to evaluate the degree of severity.
Subclinical hypothyroidism vs transient increase in TSH
The TSH dosage is the most sensitive laboratory data for the diagnosis of subclinical hypothyroidism. It must be considered, however, that some physiological or pathological situations can transiently increase the secretion of TSH.
The causes of this phenomenon include sleep disturbances, circadian rhythm abnormalities (e.g. night work), exposure to toxic substances (pesticides, industrial chemicals, etc.), some forms of thyroiditis (subacute or postpartum), antithyroid drugs or inhibiting the secretion of TSH (glucocorticoids, dopamine etc.), major surgery, severe trauma, infections and malnutrition.
based on thyroid hormone (replacement therapy with L-thyroxine, L-T4; e.g. levothyroxine), initially at low doses. The aim of the treatment is to restore a condition of euthyroidism.
Before adhering to any L-thyroxine replacement therapy, however, the physician should monitor the dysfunction in a short period of time (approximately 3-6 months) and confirm the TSH increase (it may be due to a transient abnormality) ).
If L-thyroxine is not taken (due to the patient's lack of adherence to the therapeutic protocol) or is not sufficient, a condition of hypothyroidism is created. For this reason, while taking the drug, the patient with subclinical hypothyroidism must undergo regular follow-ups to check the effects of the treatment.
Subclinical hypothyroidism: scheme for monitoring
- After the first finding of elevated TSH and normal thyroid hormones, carry out the dosage of TSH, FT4 and Anti-Thyroperoxidase antibodies (Ab anti-TPO) in the blood after 2-3 months.
- If TSH is normal, do not perform further tests;
- If TSH is high (i.e. subclinical hypothyroidism is persistent):
- Perform an ultrasound examination of the thyroid gland;
- Evaluate thyroid function every 6 months (TSH and FT4); after 2 years, this check can become annual.
In general, thyroid function should be evaluated in pregnant women, in those who develop symptoms of hypothyroidism, or in other blood chemistry tests.
Treatment of subclinical hypothyroidism: yes or no?
Even today, the treatment or not of subclinical hypothyroidism is the subject of controversy in the various guidelines.
In general, thyroid hormone replacement therapy begins when TSH values are greater than 10 µU / ml. As regards concentrations below 10 µU / ml, on the other hand, the greater stimulation of TSH on the thyroid gland tends to be exploited, so that this still ensures a normal production of thyroid hormones. Therapy can be started for TSH values between 4 and 10 µU / ml in case of chronic autoimmune thyroiditis or nodular thyroid disease.
The only condition in which the treatment of subclinical hypothyroidism is always indicated in adults is pregnancy, to avoid the effects of the dysfunction on gestation and fetal development. The initiation of therapy can be considered by the doctor in the presence of clinical symptoms or in the case of coexisting hyperlipidemia and heart failure.
.jpg)
