Page 818 - Textbook of Pathology, 6th Edition

802 anomalous lingual thyroid. The thyroglossal duct that 4. Coupling of MIT and DIT in the presence of thyroid
connects the gland to the pharyngeal floor normally peroxidase forms tri-iodothyronine (T ) and thyroxine (T ).
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disappears by 6th week of embryonic life. In adults, its The thyroid hormones so formed are released by
proximal end is represented by foramen caecum at the base endocytosis of colloid and proteolysis of thyroglobulin within
of the tongue and distal end by the pyramidal lobe of the the follicular cells resulting in discharge of T and T into
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thyroid. Persistence of the remnants of thyroglossal duct in circulation where they are bound to thyroxine-binding
the adults may develop into thyroglossal cyst (page 520). The globulin.
C-cells of the thyroid originate from the neuroectoderm. A number of thyroid function tests are currently available.
The thyroid gland in an adult weighs 15-40 gm and is These include the following:
composed of two lateral lobes connected in the midline by Determination of serum levels of T , T by radio-
a broad isthmus which may have a pyramidal lobe immunoassay (RIA). 3 4
extending upwards. Cut section of normal thyroid is
yellowish and translucent. TSH and TRH determination.
Determination of calcitonin secreted by parafollicular C
HISTOLOGY. The thyroid is composed of lobules of colloid- cells.
filled spherical follicles or acini. The lobules are enclosed by
fibrovascular septa. The follicles are the main functional units Estimation of thyroglobulin secreted by thyroid follicular
of the thyroid. They are lined by cuboidal epithelium with cells.
numerous fine microvilli extending into the follicular colloid Assessment of thyroid activity by its ability to uptake
that contains the glycoprotein, thyroglobulin. The follicles are radioactive iodine (RAIU).
separated from each other by delicate fibrous tissue that Assessment whether thyroid lesion is a nonfunctioning
contains blood vessels, lymphatics and nerves. Calcitonin- (‘cold nodule’) or hyperactive mass (‘hot nodule’).
secreting C-cells or parafollicular cells are dispersed within Diseases of the thyroid include: functional disorders
the follicles and can only be identified by silver stains and (hyperthyroidism and hypothyroidism), thyroiditis, Graves’
immunohistochemical methods. disease, goitre and tumours. The relative frequency of some
of these diseases varies in different geographic regions
FUNCTIONS. The major function of the thyroid gland is to according to the iodine content of the diet consumed. One of
maintain a high rate of metabolism which is done by means the important investigation tools available in current times
of iodine-containing thyroid hormones, thyroxine (T ) and is the widespread use of FNAC for thyroid lesions which
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tri-iodothyronine (T ). helps in avoiding a large number of unwanted diagnostic
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The thyroid is one of the most labile organs in the body
SECTION III
and responds to numerous stimuli such as puberty, biopsies.
pregnancy, physiologic stress and various pathologic states. FUNCTIONAL DISORDERS
This functional lability of the thyroid is responsible for
transient hyperplasia of the thyroidal epithelium. Under Two significant functional disorders characterised by distinct
normal conditions, the epithelial lining of the follicles may clinical syndromes are described. These are: hyperthyroidism
show changes in various phases of function as under: (thyrotoxicosis) and hypothyroidism.
1. Resting phase is characterised by large follicles lined by
flattened cells and filled with deeply staining homogeneous HYPERTHYROIDISM (THYROTOXICOSIS)
colloid e.g. in colloid goitre and iodine-treated hyper- Hyperthyroidism, also called thyrotoxicosis, is a hyper-
thyroidism. metabolic clinical and biochemical state caused by excess
Systemic Pathology
2. Secretory phase in which the follicles are lined by production of thyroid hormones. The condition is more
cuboidal epithelium and the colloid is moderately dark pink frequent in females and is associated with rise in both T
e.g. in normal thyroid. and T levels in blood, though the increase in T is generally 3
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3. Resorptive phase is characterised by follicles lined by greater than that of T . 3
columnar epithelium and containing lightly stained 4
vacuolated and scalloped colloid e.g. in hyperthyroidism. ETIOPATHOGENESIS. Hyperthyroidism may be caused
The synthesis and release of the two main circulating by many diseases but three most common causes are: Graves’
thyroid hormones, T and T are regulated by hypophyseal disease (diffuse toxic goitre), toxic multinodular goitre and
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thyroid-stimulating hormone (TSH) and involves the a toxic adenoma. Less frequent causes are hypersecretion of
following steps: pituitary TSH by a pituitary tumour, hypersecretion of TRH,
1. Iodine trapping by thyroidal cells involves absorbing of thyroiditis, metastatic tumours of the thyroid, struma ovarii,
iodine from the blood and concentrating it more than twenty- congenital hyperthyroidism in the newborn of mother with
fold. Graves’ disease, hCG-secreting tumours due to mild
2. Oxidation of the iodide takes place within the cells by a thyrotropic effects of hCG (e.g. hydatidiform mole, chorio-
thyroid peroxidase. carcinoma and testicular tumours), and lastly, by excessive
doses of thyroid hormones or iodine called jodbasedow disease.
3. Iodination occurs next, at the microvilli level between
the oxidised iodine and the tyrosine residues of thyroglobulin CLINICAL FEATURES. Patients with hyperthyroidism have
so as to form mono-iodotyrosine (MIT) and di-iodotyrosine a slow and insidious onset, varying in severity from case to
(DIT). case. The usual symptoms are emotional instability,