Astrid Libman and Julio Libman

Hypothyroidism can be defined as the condition resulting from the exposure of tissues to suboptimal levels of thyroid hormone.

The term myxedema implies those cases of advanced hypothyroidism, where there is infiltration of the skin, mucous membranes and tissues, more accentuated in the face and hands, with an extracellular metachromatic hydrophilic material composed of mucopolysaccharides, hyaluronic acid and chondroitinsulfate. The Godet sign is negative.

Subclinical hypothyroidism is defined as those asymptomatic patients with T4 and T3 concentrations within the range of statistical normality, but with elevated TSH, which indicates a decrease in thyroid functional reserve. Childhood and adolescent hypothyroidism is the condition that begins in childhood or adolescence. The term cretinism designates the picture of severe mental deficit produced by untreated neonatal hypothyroidism.

Pathophysiology

Hypothyroidism is the result of a variety of structural or functional alterations of the thyroid gland that result in a deficient production of thyroid hormones. It can rarely be due to peripheral causes, either due to resistance or a decreased tissue response to the hormone or due to the existence of antibodies that bind thyroid hormone in the circulation.

According to the place where the alteration resides, it can be classified as primary, secondary and tertiary. The term primary hypothyroidism refers to cases in which the causative lesion resides in the gland itself. It is the most common cause of thyroid insufficiency, and is much more common in females. It presents without a palpable thyroid, as in congenital atyreosis, the primary idiopathic and post-ablation form of the gland with ¹³¹I or surgery, either with an enlarged gland (goiter), as in chronic Hashimoto's thyroiditis, iodine deficiency, the passage of drugs through the placenta (iodides, antithyroid drugs) or administered in children or adults (lithium, phenylbutazone, aminoglutethimide) and by dyshormonogenesis due to the lack of any of the enzymes involved in the synthesis of thyroid hormone. In all these circumstances, and as a consequence of the decreased production of T4 and T3, pituitary TSH increases which, in cases where there is thyroid tissue, determines its hypertrophy and hyperplasia (enlarged thyroid).

The hypothyroidism secondary , much less common than the primary, rarely due to a deficiency Isolated TSH; it is commonly associated with a multiple deficiency of anterohypophyseal hormones, produced by chromophobic adenomas, postpartum ischemic necrosis (Sheehan's syndrome) or hypophysitis. Multiple deficiency can also be idiopathic.

The tertiary hypothyroidism , exceptional, hypothalamic injury involves a deficit in the production of TRH. Causes include head trauma, surgical section of the pituitary stalk, metastatic tumors, radiation, and idiopathic disease.

In hypothyroidism, as a consequence of the slowing down of a number of reactions that require energy consumption, there is a decrease in the consumption of O2 per unit of body surface area and in the rate of heat production. O2 consumption by the mitochondria and ATP formation are decreased, but oxidative phosphorylation “coupling” (the number of ATP molecules formed per O2 molecule consumed) is normal. The Na + pump, which maintains normal intra and extracellular Na + and K + gradients, is influenced by thyroid hormones. Normally, a substantial part of the O2 consumed and the formation of ATP provide energy for the Na + pump, which in hypothyroidism appears to have a diminished function. The rate of Na + removal from the cell is reduced. Thyroid administration restores O2 consumption and Na + pump activity to normal, producing an increase in activity that accounts for most of the caloric action of thyroid hormones. The decreased rate of energy utilization is clinically reflected in a drop in body temperature, intolerance to cold, a drop in cardiac output, delayed growth and maturation, and decreased basal metabolism.

Contrary to what occurs in hyperthyroidism, intolerance to low temperatures is characteristic, which makes these patients wear warm clothes in the middle of summer.

Symptoms and signs

The cutaneous manifestations are among the most striking and frequent of hypothyroidism. The skin is cold, dry, rough and covered with fine scales that come off to the touch, due to a decrease in sweat and sebaceous secretions. In secondary hypothyroidism, abnormalities of the epidermis are much less common. A characteristic infiltrate, with a negative Godet sign, more accentuated on the face and the back of the hands, is the result of an infiltration of the dermis by an extracellular fluid very rich in polysaccharides. When it affects the tongue, it is a cause of macroglossia. Except in the malar region, where there may be some redness of uncertain cause, the skin has a pale waxy color, caused by concomitant anemia, peripheral vasoconstriction to reduce heat loss and increased skin thickness due to myxedematous infiltration, which prevents visualization of the dermal capillary network. The yellowish tone that can be observed in long-standing hypothyroidism is due to an increase in plasma and tissue carotene, which results in part from a blockage of its metabolism to vitamin A. Hair is dry, coarse and brittle. Alopecia occurs in a fifth of patients, due to decreased hair growth and increased fragility of the inner sheath of the hair root. Axillary and pubic hair is sparse, and is practically absent in secondary hypothyroidism due to the concomitant deficit of sex steroids. As opposed, Juvenile hypothyroidism patients often have long, dark hair on the back and extremities. Although loss of the outer third of the eyebrows is described as characteristic of hypothyroidism in adults, this sign can be seen in large numbers of elderly euthyroid individuals. Nails become thin and brittle, and develop transverse grooves in most patients with myxedema.

Cerebral metabolism is abnormal, and there is a decrease in blood flow and consumption of O2 and glucose and an increase in vascular resistance. The electroencephalogram may be altered and show absence of the normal rhythm with the appearance of slow waves with a frequency of 3 to 6 per second. These abnormalities can explain the clinical manifestations, which are translated into a decrease in psychic, sensory and motor functions. Initially, the manifestations are nonspecific, such as asthenia, drowsiness, difficulty in learning, attention and concentration, slowing of thought and speech, and a hoarse voice due to myxedematous infiltration of the vocal cords. In more advanced cases, the patient is isolated from the environment, and a hypothyroid coma may develop. Seizures are possible, dysarthria, cerebellar ataxia and a picture of psychosis. There is an increase in proteins in the CSF, probably due to an alteration in capillary permeability.

Sporadic cretinism and endemic goitre cretinism differ from adult myxedema by their severe and characteristic mental deficit and marked delay in maturation and physical development.

Signs of peripheral neuropathy are equally common in long-standing hypothyroidism. Patients with myxedema may present paresthesias in the territory of the median nerve, with pain on percussion of the median nerve (Tinel's sign) or when flexing the wrist (Phalen's sign), characteristic of carpal tunnel syndrome, produced by compression of the nerve at the level of this anatomical structure by swollen and infiltrated tissues. Patients may also present with lower limb paresthesia, as well as hyporeflexia with a delay in the relaxation phase of deep tendon reflexes (eg, the Achilles). Of the cranial nerves, the most affected is the auditory component of the 8th pair. Deafness or hearing loss may be due to nerve or conduction disturbances.

Patients with severe hypothyroidism commonly present with dyspnea and easy fatigue. Cardiovascular physical examination reveals bradycardia, weak pulses, normal or slightly elevated blood pressure, distant heart sounds, a quiet precordial zone (as opposed to the erethism seen in hyperthyroidism), and evidence of cardiomegaly. Jugular pressure is generally normal.

Pericardial effusions are common, with a high protein content due to greater capillary permeability. Because they occur slowly, cardiac tamponade occurs rarely. The ECG reveals low-voltage complexes. The chest radiograph may show an enlarged cardiac silhouette, often from a pericardial effusion.

Myxedema almost never causes heart failure in the absence of other heart diseases. Coronary artery disease, on the other hand, occurs more frequently than in euthyroid patients due to concomitant alterations in lipid metabolism, and can become clinically apparent when starting replacement therapy.

Hemodynamic studies reveal a decrease in minute volume, stroke volume, rate, and vital capacity.

Peripheral vasoconstriction, together with decreased cardiac output, results in normal or slightly increased peripheral resistance and an equally unchanged or slightly elevated blood pressure. The observed alterations appear to be a consequence of the direct action of the lack of thyroid hormones on the heart, as well as an indirect effect of the reduced O2 requirements of both the heart and the peripheral tissues.

Constipation, an expression of intestinal hypomotility, is a frequent symptom. The most serious complication of hypomotility of the gastrointestinal tract is paralytic ileus, probably due to myxedematous infiltration of the walls of the intestine. It is possible to observe dyspepsia and achlorhydria, with anti-gastric mucosa antibodies, as well as recurrent ascites with exudate characteristics.

Muscles may be enlarged, contract slowly, and tend to be weaker than in normal individuals. In response to pinching, for example, at the level of the biceps, a visible and palpable muscle roll is formed that lasts for several seconds. Although it is a frequent sign, it is not pathognomonic (a sign of myoedema). There may be nonspecific joint symptoms, including strokes and pain of varying intensity.

Hypothyroidism delays the growth and maturation of the skeleton. The hypothyroid child maintains skeletal proportions corresponding to a younger chronological age, with disproportionately short limbs. The ossification of the epiphyseal nuclei and of the short bones is delayed (delayed bone age), with Ca ++ depositing irregularly, with a mottled or fragmented radiological appearance (epiphyseal dysgenesis).

Menstrual irregularities are frequent, with a tendency to polyhypermenorrhea. In secondary hypothyroidism, amenorrhea is common, due to the simultaneous deficiency of gonadotropins.

Study methodology

Given the existence of symptoms and signs suggestive of hypothyroidism, complementary studies are used to confirm the diagnosis and determine its primary or secondary nature. In this sense, and from the point of view of the interrogation and physical examination, the existence of a history of thyroid surgery or the administration of ¹³¹ I for the treatment of previous hyperthyroidism, the presence of goiter, the absence of a history that suggests a Sheehan's syndrome or manifestations that indicate a pituitary tumor syndrome or deficiencies of other hormones, generally point to the diagnosis of primary hypothyroidism.

Total plasma thyroxine dose (T4). Normal concentrations range from 5 to 12 ug%. It is decreased in both primary and secondary hypothyroidism, as well as in other circumstances characterized by a decrease in thyroxine-binding globulin (TBG) (genetics, administration of androgens and anabolics that inhibit its hepatic synthesis, nephrotic syndrome with protein loss with urine) or in which there is a displacement of the binding of T4 to TBG, as during the administration of diphenylhydantoin.

Total plasma triiodothyronine dosage (T3). It is diminished; normal concentrations range from 70 to 220 ng / ml. It is important to remember that a large proportion of circulating T3 is derived from peripheral deiodination in the tissues of T4. In the presence of severe non-thyroid diseases, T4 is preferentially transformed into a less metabolically active compound, reverse T3, which is the cause of low levels of T3 in plasma. These changes constitute the syndrome of euthyroidism with low T3.
Investigation of antithyroid antibodies. They are of two types: anti-peroxidase and anti-thyroglobulin. They are normally present in low concentrations. A marked increase is characteristic of primary hypothyroidism due to chronic Hashimoto's thyroiditis.

Thyrotrophin (TSH) dosage. Normal concentrations vary between 0.5 and 5.0 uU / ml. It is clearly increased in primary hypothyroidism. The existence of TSH within the normal range, together with low T4 and T3, indicate the probable presence of secondary hypothyroidism.

Uptake of ¹³¹ I . It is of little use in the diagnosis of hypothyroidism

Other studies . As complementary and non-specific data, hyperlipidaemia, mainly type IIa (hypercholesterolemia), and anemia are observed; In children, a bone age behind chronological age is detected through X-rays of the hand and wrist, tarsus and epiphyses of long bones.