|Classification and external resources|
|Specialty||endocrinology, medical genetics|
A thyroid disease is a medical condition impairing the function of the thyroid. Different thyroid diseases include thyroiditis, hyperthyroidism and hypothyroidism. These diseases have a large range of symptoms and affect all ages.
- 1 Diseases
- 2 Diagnosis
- 3 Treatment
- 4 References
- 5 Further reading
- 6 External links
Imbalances in the production of thyroid hormones arise from dysfunction of either 1) the thyroid gland itself, 2) the pituitary gland, which produces thyroid-stimulating hormone (TSH), or 3) the hypothalamus, which regulates the pituitary gland via thyrotropin-releasing hormone (TRH). Concentrations of TSH increase with age, requiring age-corrected tests. Hypothyroidism affects 3-10% percent of adults, with a higher incidence in women and the elderly.
Low function: Hypothyroidism
- Postoperative hypothyroidism
- Iatrogenic hypothyroidism[clarification needed]
- Thyroid hormone resistance
- Euthyroid sick syndrome
High function: Hyperthyroidism
- Graves' disease
- Toxic thyroid nodule
- Thyroid storm
- Toxic nodular struma (Plummer's disease)
- Thyroid adenoma
- Thyroid cancer
- Lymphomas and metastasis from elsewhere (rare)
Thyroid Function Tests
Further information: Thyroid function tests
There are several hormones that can be measured in the blood to determine how the thyroid gland is functioning. These include the thyroid hormones triiodothyronine (T3) and its precursor thyroxine (T4), which are produced by the thyroid gland. Thyroid-secreting hormone (TSH) is another important hormone that is secreted by the anterior pituitary cells in the brain. Its primary function is to increase the production of T3 and T4 by the thyroid gland.
The most useful marker of thyroid gland function is serum thyroid-stimulating hormone (TSH) levels. TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal.
- Elevated TSH levels can signify inadequate thyroid hormone production (hypothyroidism)
- Suppressed TSH levels can point to excessive thyroid hormone production (hyperthyroidism)
Because a single abnormal TSH level can be misleading, T4 and T3 levels must be measured in the blood to further confirm the diagnosis. When circulating in the body, T4 and T3 are bound to transport proteins, namely thyroxine-binding globulin, transthyretin and albumin. Only a small fraction of the circulating thryoid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as what is called free T3 and T4, or total T3 and T4, which takes into consideration the free T3/T4 in addition to the protein-bound T3/T4. Free T3 and T4 measurements are important because certain drugs and illnesses can affect the concentrations of transport proteins, resulting in differing total and free thyroid hormone levels.
- Free T4 levels are often added to determine the degree of hypothyroidism when disease is suspected in the hypothalamus or pituitary gland
- Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH
- Autoantibodies may be detected in various disease states (anti-TG, anti-TPO, TSH receptor stimulating antibodies).
- There are two cancer markers for thyroid derived cancers. Thyroglobulin (TG) for well-differentiated papillary or follicular adenocarcinoma, and the rare medullary thyroid cancer has calcitonin as the marker.
- Very infrequently, TBG and transthyretin levels may be abnormal; these are not routinely tested.
- To differentiate between different types of hypothyroidism, a specific test may be used. Thyrotropin-releasing hormone (TRH) is injected into the body through a vein. This hormone is naturally secreted by the hypothalamus and stimulates the pituitary gland. The pituitary responds by releasing thyroid -stimulating hormone (TSH). Large amounts of externally administered TRH can suppress the subsequent release of TSH. This amount of release-suppression is exaggerated in primary hypothyroidism, major depression, cocaine dependence, amphetamine dependence and chronic phencyclidine abuse. There is a failure to suppress in the manic phase of bipolar disorder.
Thyroid nodules may or may not be cancer. Medical ultrasonography can help determine their nature because some of the characteristics of benign and malignant nodules differ. The main characteristics that can distinguish a benign vs. malignant thyroid nodule on high frequency thyroid ultrasound are as follows:
|Possible cancer||Benign characteristics|
|irregular borders||smooth borders|
|hypoechoic (less echogenic than the surrounding tissue)||hyperechoic|
|significant intranodular blood flow by power Doppler||"comet tail" artifact as sound waves bounce off intranodular colloid|
|taller than wide shape on transverse study|
Ultrasonography is not always able to separate benign from malignant nodules with certainty. In suspicious cases, a tissue sample is often obtained by biopsy for microscopic examination.
Radioiodine scanning and uptake
Thyroid scintigraphy, imaging of the thyroid with the aid of radioactive iodine, usually iodine-123 or iodine-131, is performed in the nuclear medicine department of a hospital or clinic. Radioiodine collects in the thyroid gland before being excreted in the urine. While in the thyroid the radioactive emissions can be detected by a camera, producing a rough image of the shape (a radiodine scan) and tissue activity (a radioiodine uptake) of the thyroid gland.
A normal radioiodine scan shows even uptake and activity throughout the gland. Irregularity can reflect an abnormally shaped or abnormally located gland, or it can indicate that a portion of the gland is overactive or underactive, different from the rest. For example, a nodule that is overactive ("hot") to the point of suppressing the activity of the rest of the gland is usually a thyrotoxic adenoma, a surgically curable form of hyperthyroidism that is hardly ever malignant. In contrast, finding that a substantial section of the thyroid is inactive ("cold") may indicate an area of non-functioning tissue such as thyroid cancer.
The amount of radioactivity can be counted as an indicator of the metabolic activity of the gland. A normal quantitation of radioiodine uptake demonstrates that about 8-35% of the administered dose can be detected in the thyroid 24 hours later. Overactivity or underactivity of the gland as may occur with hypothyroidism or hyperthyroidism is usually reflected in decreased or increased radioiodine uptake. Different patterns may occur with different causes of hypo- or hyperthyroidism.
A medical biopsy refers to the obtaining of a tissue sample for examination under the microscope or other testing, usually to distinguish cancer from noncancerous conditions. Thyroid tissue may be obtained for biopsy by fine needle aspiration or by surgery.
Needle aspiration has the advantage of being a brief, safe, outpatient procedure that is safer and less expensive than surgery and does not leave a visible scar. Needle biopsies became widely used in the 1980s, but it was recognized that accuracy of identification of cancer was good but not perfect. The accuracy of the diagnosis depends on obtaining tissue from all of the suspicious areas of an abnormal thyroid gland. The reliability of needle aspiration is increased when sampling can be guided by ultrasound, and over the last 15 years, this has become the preferred method for thyroid biopsy in North America.
Levothyroxine is a stereoisomer of thyroxine which is degraded much slower and can be administered once daily in patients with hypothyroidism. Natural thyroid hormone from pigs is also used, especially for people who cannot tolerate the synthetic version. Graves' disease may be treated with the thioamide drugs propylthiouracil, carbimazole or methimazole, or rarely with Lugol's solution. Hyperthyroidism as well as thyroid tumors may be treated with radioactive iodine.
Thyroid surgery is performed for a variety of reasons. A nodule or lobe of the thyroid is sometimes removed for biopsy or for the presence of an autonomously functioning adenoma causing hyperthyroidism. A large majority of the thyroid may be removed, a subtotal thyroidectomy, to treat the hyperthyroidism of Graves' disease, or to remove a goitre that is unsightly or impinges on vital structures.
A complete thyroidectomy of the entire thyroid, including associated lymph nodes, is the preferred treatment for thyroid cancer. Removal of the bulk of the thyroid gland usually produces hypothyroidism, unless the person takes thyroid hormone replacement. Consequently, individuals who have undergone a total thyroidectomy are typically placed on thyroid hormone replacement for the remainder of their lives. Higher than normal doses are often administered to prevent recurrence.
If the thyroid gland must be removed surgically, care must be taken to avoid damage to adjacent structures, the parathyroid glands and the recurrent laryngeal nerve. Both are susceptible to accidental removal and/or injury during thyroid surgery. The parathyroid glands produce parathyroid hormone (PTH), a hormone needed to maintain adequate amounts of calcium in the blood. Removal results in hypoparathyroidism and a need for supplemental calcium and vitamin D each day. In the event the blood supply to any one of the parathyroid glands is endangered through surgery, the parathyroid gland(s) involved may be re-implanted in surrounding muscle tissue. The recurrent laryngeal nerves provide motor control for all external muscles of the larynx except for the cricothyroid muscle, which also runs along the posterior thyroid. Accidental laceration of either of the two or both recurrent laryngeal nerves may cause paralysis of the vocal cords and their associated muscles, changing the voice quality.
Large goiters that cause symptoms but do not harbor cancer, after evaluation and biopsy of suspicious nodules, can be treated by an alternative therapy with radioiodine. The iodine uptake can be high in countries with iodine deficiency, but low in iodine sufficient countries. The 1999 release of recombinant human TSH, Thyrogen, in the USA, can boost the uptake to 50-60% allowing the therapy with Iodine 131. The gland shrinks by 50-60% but can cause hypothyroidism and rarely pain syndrome, which arises due to radiation thyroiditis. It is short lived and treated by steroids.
- Surks MI, Hollowell JG (December 2007). "Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population: implications for the prevalence of subclinical hypothyroidism". J. Clin. Endocrinol. Metab. 92 (12): 4575–82. doi:10.1210/jc.2007-1499. PMID 17911171.
- Gharib H, Tuttle RM, Baskin HJ, Fish LH, Singer PA, McDermott MT (2004). "Subclinical thyroid dysfunction: a joint statement on management from the American Association of Clinical Endocrinologists, the American Thyroid Association, and the Endocrine Society". Endocr Pract. 10 (6): 497–501. doi:10.4158/ep.10.6.497. PMID 16033723.
- Fatourechi V (2009). "Subclinical hypothyroidism: an update for primary care physicians". Mayo Clin. Proc. 84 (1): 65–71. doi:10.4065/84.1.65. PMC . PMID 19121255.
- Villar HC, Saconato H, Valente O, Atallah AN (2007). Villar HC, ed. "Thyroid hormone replacement for subclinical hypothyroidism". Cochrane Database Syst Rev (3): CD003419. doi:10.1002/14651858.CD003419.pub2. PMID 17636722.
- Jameson, J; et al. (2015). Harrison's Principal of Internal Medicine, 19e. New York: NY: McGraw-Hill. pp. Ch 405 – via Access Medicine.
- Giannini AJ, Malone DA, Loiselle RH, Price WA (1987). "Blunting of TSH response to TRH in chronic cocaine and phencyclidine abusers". J Clin Psychiatry. 48 (1): 25–6. PMID 3100509.
- Brent, Gregory A. (Ed.), Thyroid Function Testing, New York : Springer, Series: Endocrine Updates, Vol. 28, 1st Edition., 2010. ISBN 978-1-4419-1484-2
- Medline Plus Medical Encyclopedia entry for Thyroid Disease
- MyThyroid.com An evidence-based patient-centered website devoted to diseases of the thyroid
- Thyroid disease portal at About.com managed by health activist and patient advocate Mary Shomon
- Thyroid Disease Manager A group of thyroid experts offer an up-to-date analysis of all aspects of human thyroid disease and thyroid physiology
- Medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism from the American Association of Clinical Endocrinologists
- Clinical Thyroidology - Free online monthly magazine The Editor searches for the best new thyroid studies, summarizes them and provides expert commentary
- Clinical Thyroidology - Patient version The more accessible version of the magazine Clinical Thyroidology, directed at the patient