Hypothyroidism resident survival guide

Hypothyroidism Resident Survival Guide Microchapters
Overview
Causes
Diagnosis
Treatment
Do's
Don'ts

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ayeesha Kattubadi, M.B.B.S[2]

Synonyms and keywords:

Overview

Hypothyroidism is a clinical state where there is a reduced production or action of thyroid hormones - Thyroxine(T4) and Triiodothyronine(T3). It is classified based on the location of pathology into primary hypothyroidism, secondary hypothyroidism and tertiary hypothyroidism. The most common cause worldwide is Iodine deficiency. In iodine sufficient areas the most common cause is autoimmune thyroiditis. Risk factors for the development of hypothyroidism include female sex, iodine deficiency, selenium deficiency, presence of other autoimmune conditions. Clinical presentation depends on the degree and rapidity of hormone depletion. Patients can be asymptomatic or present with symptoms like cold intolerance, fatigue, weight gain, constipation, depression, neck mass. The diagnosis of hypothyroidism is made with the help of biochemical tests measuring TSH and Thyroxine(FT4) levels. Treatment of this condition is with Levothyroxine supplementation.

Causes

Hypothyroidism can be classified based on the location of the pathology or based on etiology as shown in the table below.

Life Threatening Causes

• Myxedema coma

Common Causes

• Primary hypothyroidism:
  • Iodine deficiency,
  • Autoimmune thyroiditis.
• Central hypothyroidism:
  • Pituitary macroadenoma,
  • Iatrogenic (Pituitary surgery, radiation).
• Congenital hypothyroidism:
  • Iodine deficiency,
  • Thyroid dysgenesis.

Causes of hypothyroidism
Primary hypothyroidism					Central hypothyroidism ( Secondary and Tertiary )							Congenital hypothyroidism
Auto immune	Drug induced	Iatrogenic	[[Transient Hypothyroidism]]	Infiltrative disorders	Genetic causes	Iatrogenic	Tumors	Vascular	Infiltrative	Infectious	Miscellaneous
Hashimoto's thyroiditis Atrophic thyroiditis	Lithium Thionamides Amiodarone Iodine containing contrast agents Interferon - alfa Tyrosine kinase inhibitor	Iodine deficiency Subtotal or Total thyroidectomy Radioiodine therapy External radiation of the neck	Subacute granulomatous thyroiditis Postpartum thyroiditis Painless (silent, lymphocytic) thyroiditis	Amyloidosis Sarcoidosis Hemochromatosis Scleroderma Cystinosis Reidel's thyroiditis( fibrous thyroiditis )	TRH deficiency Isolated CeH Inactive TSH Inactive TRH receptor	Post pituitary surgery Post external radiation Drugs: Growth hormone therapy, Glucocorticoids, Retinoid X Receptor(RXR) agonists, Salicylates, Dopamine	Pituitary macroadenoma Craniopharyngioma Meningioma Glioma Rathke cleft cyst Metastatic	Hemorrhage: Pituitary apoplexy, Subarachnoid hemorrhage Ischemia: Postpartum pituitary necrosis Aneurysm	Hemochromatosis Histiocytosis Sarcoidosis	Syphilis Bacterial abscess Tuberculosis Toxoplasmosis	Transient: overreplacement of T4 in primary hypothyroidism, Sick euthyroid state Trauma: head injury	Iodine deficiency Thyroid dysgenesis Thyroid agenesis Thyroid dyshormonogenesis

Diagnosis

Shown below is an algorithm summarizing the diagnosis of hypothyroidism </nowiki>

Signs and Symptoms of hypothyroidism are present.

TSH, free T4 (FT4)

TSH >5.5mU/L

TSH 0.5-5mU/L

TSH<0.5mU/L

FT4 Low

FT4 Normal

FT4 Elevated

FT4 Low

FT4 Low

TPOAb (Thyroid peroxidase antibody)

Subclinical hypothyroidism

1. T4 to T3 conversion defect (5' deiodinase deficiency, amiodarone use), 2. Thyroid hormone resistance

1. Central hypothyroidism, 2. T3 replacement therapy

1. Following excess levothyroxine withdrawal, 2. "Post hyperthyroid" hypothyroidism ( I131 or surgery), 3. T3 replacement therapy, 4.Central hypothyroidism

Positive

Negative

Central hypothyroidism

Autoimmune thyroid disease (Hashimotos disease)

1. Euthyroid sick syndrome 2. External radiation 3. Drug induced 4. Iodine deficiency 5. Congenital hypothyroidism 6. Seronegative autoimmune thyroid disease

Brain MRI

Abnormal

Normal

Pituitary or Hypothlamus lesion

Congenital TRH, TSH deficiency, Infiltrative diseases

Treatment

Shown below is an algorithm summarizing the treatment of Primary hypothyroidism.

Primary hypothyroidism

TSH > 5.5mU/L, FT4 Low

TSH = 5.5 - 10 mU/L, FT4 Normal

Start Levothyroxine

Symptoms of hypothyroidism

Measure TSH after 4-6 weeks

Present

Absent

Goal TSH = 0.4-4mU/L Adjust levothyroxine dose if goal TSH not reached

Levothyroxine trial for 3-6 months.

TPoAB (Thyroid Peroxidase) Antibodies

Symptoms of hypothyroidism

Positive

Negative

Resolved

Unresolved

Follow up with annual TSH

Follow up with TSH every 3 years

Lifelong levothyroxine therapy

Shown below is an algorithm summarizing the treatment of Central hypothyroidism.

Central hypothyroidism

TSH<0.5mU/L 0r 0.5-5.5mu/L, FT4 Low

Adrenal insufficiency

No

Yes

MRI Brain

Give glucocorticoids for 1-2 weeks to prevent adrenal crisis

Tumor

Levothyroxine

Present

Absent

Consider Surgery

Levothyroxine

Reassess by measuring FT4 Goal FT4 is upper limit of normal

Do's

• When to take Levothyroxine: Morning 30-60 minutes before breakfast or 4 hours after last meal of the day. To be taken at the same time each day with water.
• After initiation or change in dose of Levothyroxine in a patient diagnosed with hypothyroidism, serum TSH measurements should be done after 4-8 weeks. Once the target TSH level is reached, the TSH level is to be repeated after 6 months, thereafter every 12 months.
• In patients who are started on drugs that alter the absorption or metabolism of Levothyroxine, TSH should be measured within 4-8 weeks for dose adjustments.
• A patient may resume using the previously used full replacement dose of Levothyroxine after an interruption lasting less than 6 weeks if there was no cardiac event or weight loss in the interim.
• In a patient with subclinical hypothyroidism with TSH levels between the upper limit of normal and 10mIU/L treatment should be considered if -
  • Patient is symptomatic,
  • Positive for TPOAb,
  • Evidence or association with risk factors for atherosclerotic cardiovascular disease, heart failure.
• In patients with central hypothyroidism associated with adrenal insufficiency, glucocorticoid therapy should be given prior to starting Levothyroxine.
• In patients with central hypothyroidism FT4 levels guide therapy - target level should be greater than mid-normal range value.

Don'ts

• Serum FT3 or Total T3 levels – NOT to be used to diagnose hypothyroidism.
• In hospitalized patients, TSH measurements should not be done to assess thyroid function unless there is a high index of suspicion for thyroid disease.
• Cholesterol levels, muscle enzymes, reflex relaxation time - cannot be used as diagnostic tools for hypothyroidism.
• Thyroid replacement therapy NOT to be prescribed without biochemical evidence of hypothyroidism.
• Do not use thyroid replacement therapy for weight loss in obesity if the patient is euthyroid.
• Iodine supplementation/ iodine-rich foods should not be used in iodine-sufficient areas for hypothyroidism.
• Do not use Selenium for preventing or treating hypothyroidism.
• Do not use desiccated thyroid hormone for the treatment of hypothyroidism.

References

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