#78: Congenital Hypothyroidism

February 15, 2023 | By Sam Masur

Audio

Summary

Interpreting thyroid function tests got you feeling tired? Join us for an exciting episode with Dr. Meghan Fredette as she guides us through the ups and downs of thyroid hormone regulation and the management of congenital hypothyroidism. This is GOIT-er be a good one!

Congenital Hypothyroidism Pearls

1. Hypothyroidism refers to low thyroid hormone levels that can be secondary to either inadequate thyroid hormone production from the thyroid gland (primary hypothyroidism) or a defect in the production of TSH at the level of the hypothalamus or pituitary gland (central hypothyroidism). 
2. The most common cause of hypothyroidism in newborns is thyroid dysgenesis, while autoimmune etiologies are responsible for the majority of acquired cases of hypothyroidism in older children. 
3. It is important to act on abnormal newborn thyroid screening immediately as delays in the diagnosis and treatment of congenital hypothyroidism can lead to impaired neurocognitive outcomes. 
4. Levothyroxine is the mainstay of treatment for hypothyroidism and requires close monitoring of TSH and free T4 to ensure adequate dosing.

Congenital Hypothyroidism Notes

Congenital Hypothyroidism: Low thyroid hormone levels

• Primary: Dysfunction at the level of the thyroid; High TSH, low free T4.
• Central: Problem at the level of the pituitary or hypothalamus; Low/normal TSH, low free T4. 

Thyroid Function Tests: TSH and free T4 most commonly used

• TSH: Most sensitive screen to evaluate for thyroid disease.
• Free T4: Main thyroid hormone in the blood. 
• Total T4: T4 bound to carrier proteins, not as reliable as free T4 because influenced by total protein levels (i.e nephropathy, thyroglobulin deficiency) and estrogen levels (i.e taking OCP’s).  
• T3: Active thyroid hormone, mostly used to evaluate hyperthyroidism.
• Free T3: Free active hormone, not usually used diagnostically.

Causes of Hypothyroidism

CONGENITAL HYPOTHYROIDISM 

• Thyroid dysgenesis: Most common cause, problem with formation or migration of the thyroid gland.
• Thyroid dyshormonogenesis: Defect in thyroid hormone synthesis, secretion, or metabolism.
• Central: Defect in production of TSH due to hypothalamic or pituitary dysfunction, usually associated with other pituitary hormone deficiencies. 
• Transient: Due to maternal antithyroid drugs, maternal blocking antibodies, iodine deficiency, or iodine exposure. 

ACQUIRED HYPOTHYROIDISM 

• Autoimmune: Most common cause in adolescents.
• Iodine deficiency or excess .
• Drugs: Antithyroid dugs, AED’s, lithium .
• Thyroid Injury: Radiation therapy, infiltrative disease. 

Diagnosis 

Labs: TSH, Free T4 

Imaging can be considered in select cases:

• Thyroid US:  To identify thyroid shape, size, and location. 
• Thyroid radionuclide uptake scan: Use if US does not detect thyroid in proper location.
• Brain MRI: If central hypothyroidism.

Newborn Screening: 

Types: Most based on free T4 (with reflex to TSH) or TSH. 

• Note: TSH-based screens can miss central hypothyroidism! 

Timing: Best to draw 1-2 days after birth. 

• Repeat screening required for premature infants as development of thyroid axis can lead to false positives. 

Management: 

• Mild (TSH 20-30mIU/L): Repeat serum TSH and free T4.
• Severe (TSH >30mIU/L ): Call Endocrinologist and start levothyroxine immediately. 

Management of Congenital Hypothyroidism: 

Dosing: Start with levothyroxine 10-15mcg/kg/day. 

Monitoring: Check free T4 and TSH 2 weeks after initiation, every 2 months for first 6 months, every 3-4 months until 3 years of age, and every 6 months after age 3.

• Can consider trial off of levothyroxine at 3 years of age. 
• If any dose change, recheck in 4-6 weeks to see impact.

Administration:

• Given as a crushed tablet with formula/milk/water.  Liquid formulation is not reliable. 
• Do not give with iron, calcium, or soy containing products as they decrease absorption. 

Thyroid FAQ’s: 

• How do you manage abnormal TSH in acutely ill patients? Avoid thyroid studies in sick patients if you can!  Patients can develop sick euthyroid syndrome, which looks like central hypothyroidism (low T4 and T3 , low/normal TSH). If thyroid studies are obtained, repeat in several weeks to ensure recovery. 

• What is subclinical hypothyroidism and should it be treated? Elevated TSH with normal free T4. Typically asymptomatic. Usually not treated unless TSH>10. 

• Side effects of levothyroxine? Generally few side effects. Can develop school/behavioral issues, pseudotumor cerebri, bone age advancement.

• How does pregnancy affect TSH level and management? Increased thyroid hormone requirements at the start of pregnancy which require higher levothyroxine dosing.

Goal

Listeners will explain the basic definition, pathophysiology, diagnosis, and management of congenital hypothyroidism 

Learning Objectives

After listening to this episode listeners will be able to…  

1. Describe the normal regulation of the hypothalamic-pituitary-thyroid axis.
2. Interpret thyroid function testing and differentiate between primary and central hypothyroidism.
3. Develop a differential for the most common causes of pediatric hypothyroidism by age group.
4. Explain the approach to initiation of levothyroxine therapy and monitoring of children with congenital hypothyroidism.

Disclosures

Dr. Fredette reports no relevant financial disclosures. The Cribsiders report no relevant financial disclosures. 

Citation

Ivanova M, Fredette M, Cruz M, Masur S, Chiu C, Berk J. “#78: Congenital Hypothyroidism”. The Cribsiders Pediatric Podcast. https:/www.thecribsiders.com/  February 15, 2023.