Disorders of the

Thyroid Gland

By Amir Ashkan Ashrafian M.D

Disorders of the Thyroid Gland: Introduction

The thyroid gland ⇨ T3 & T4⇨Thyroid hormone receptors ell differentiation + Thermogenic & metabolic homeostasis

Autoimmune disorders: overproduction of thyroid hormones

(thyrotoxicosis) OR Glandular destruction and hormone

deficiency (hypothyroidism) Benign nodules and various forms of

thyroid cancer

Thyroid Anatomy The thyroid (Greek thyreos, shield, plus

eidos, form) consists of two lobes connected by isthmus

Anterior to the trachea between the cricoid cartilage and the suprasternal notch

Normal thyroid is 12–20 g , vascular, soft

Four parathyroid glands,producing parathyroid hormone , are located posterior to each pole of the thyroid

Thyroid Histology

The thyroid gland consists of numerous spherical follicles composed of thyroid follicular cells that surround secreted colloid, a proteinaceous fluid containing large amounts of thyroglobulin, the protein precursor of thyroid hormones

Regulation of the Thyroid Axis

TSH, secreted by the thyrotrope cells of the anterior pituitary, pivotal role in controling thyroid axis

The most useful physiologic marker of thyroid hormone action

The thyroid axis is a classic example of an endocrine feedback loop. Hypothalamic TRH stimulates pituitary production of TSH, which, in turn, stimulates thyroid hormone synthesis and secretion. Thyroid hormones feed back to inhibit TRH and TSH production

Iodine Metabolism and Transport

Iodide uptake is a critical first step in thyroid hormone synthesis. Ingested iodine is bound to serum proteins, particularly albumin. Unbound iodine is excreted in the urine. The thyroid gland extracts iodine from the circulation in a highly efficient manner

Cretinism

Cretinism : mental & growth retardation Children in iodine-deficient regions who not treated

with iodine or thyroid hormone (early life). These children are often born to mothers with iodine deficiency

∼Maternal T3,T4 deficiency worsens the condition Concomitant selenium def. ⤳ neurologic

manifestations of Cretinism Iodine supplement of salt, bread etc. ⇨⬇Cretinism. Iodine deficiency the most common cause of

preventable mental deficiency Mild iodine deficiency ⟾ subtle reduction of IQ Oversupply of iodine ⤳ ⬆Autoimmune thyroid

disease

Hypothyroidism

Iodine deficiency remains the most common cause of hypothyroidism worldwide. In areas of iodine sufficiency, autoimmune disease (Hashimoto's thyroiditis) and iatrogenic causes (treatment of hyperthyroidism) are most common

Classification Congenital Hypothyroidism

Iatrogenic hypothyroidism

Autoimmune Hypothyroidism ⥥ Hashimoto's ( Goitrous thyroiditis ) ⬇ Atrophic thyroiditis

Clinical Manifestations Insidious onset : may become aware of

symptoms only after euthyroidism Hashimoto's thyroiditis ⤳ irregular firm

goiter rather than symptoms of HypoT Dry skin, decreased sweating Puffy face, edematous eyelids Nonpitting pretibial edema Constipation & weight gain (despite

poor appetite) Due to fluid retention in the

myxedematous tissues

Clinical Manifestations

There is pallor, & often yellow tinge to the skin due to carotene accumulation

Nail growth is retarded Hair is dry, brittle, difficult to manage,

and falls out easily ( diffuse alopecia ) Thinning of the outer third of the

eyebrows (Also in Leprosy) Cool extremities Impaired Memory & concentration

Associated Autoimmune diseases

Vitiligo

Pernicious anemia

Addison's disease

Alopecia areata

Type 1 diabetes mellitus

Treatment: Hypothyroidism

Daily replacement dose of levothyroxine

Radiation

Early studies of the pathogenesis of thyroid cancer focused on the role of external radiation, which predisposes to chromosomal breaks, leading to genetic rearrangements and loss of tumor-suppressor genes.

Thyrotoxicosis Thyroid hormone excess ⤷HyperthyroidismCauses of Thyrotoxicosis: Primary hyperthyroidism Graves' disease(Genetic + Iodine

intake) Toxic multinodular goiter Toxic adenoma Drugs: iodine excess (Jod-Basedow

phenomenon)

Symptoms of Thyrotoxicosis Hyperactivity, irritability, dysphoria

Heat intolerance and sweating

Palpitations

Fatigue and weakness

Weight loss with increased appetite

Diarrhea

Polyuria

Oligomenorrhea, loss of libido

Signs of Thyrotoxicosis Tachycardia; atrial fibrillation in the elderly

Tremor

Goiter

Warm, moist skin

Muscle weakness, proximal myopathy

Lid retraction or lag

Gynecomastia

Treatment: Graves' Disease

By reducing thyroid hormone synthesis

Using antithyroid drugs

Reducing the amount of thyroid tissue with radioiodine (131I) treatment

Thyroidectomy