Chapter 2: ENDOCRINOLOGY

THYROID DISORDERS—KEY TERMINOLOGY

DEFINITIONS of Thyroglobulin, Thyroxine-Binding Globulin, Thyroxine, and Free T4

• ­THYROGLOBULIN: This protein is found only in the thyroid It is used to make thyroid hormones.
• ­THYROXINE-BINDING GLOBULIN; TBG is responsible for carrying thyroid hormones in the blood.
• ­THYROXINE: THYROXINE = T4 = Hormone that is BOUND to TBG and in the blood and INACTIVE
• ­FREE T4: FREE T4 = FT4 = the ACTIVE hormone in the blood

HYPOTHYROIDISM

THYROXINE-BINDING GLOBULIN DEFICIENCY

Thyroxine-binding globulin deficiency is an X-linked disease in which the newborn screen (NBS) reveals normal TSH but LOW T4 values. NBS is measuring T4 (BOUND hormone), which is low because there is a TBG deficiency! Get a FREE T4 as the next step. It should be NORMAL. Another option is to get a TBG level, which should be low. NO treatment is indicated. So, if TSH is normal but T4 is low on NBS → NO TREATMENT.

PEARL: TSH is the barometer for the thyroid. If it’s normal, there’s probably no clinical problem.

HYPOTHYROIDISM & CONGENITAL HYPOTHYROIDISM

Hypothyroidism and congenital hypothyroidism are diagnosed with elevated TSH and a low Free T4 (get T4 if FT4 not offered on exam). If asked how to screen, choose TSH. In utero, some of mom’s thyroxine crosses the placenta so babies may by asymptomatic at birth. Signs of congenital hypothyroidism may include lethargy, constipation, dry skin, puffiness, large tongue, hoarse cry, umbilical hernia, hypotonia, large anterior fontanelle (AF), open posterior fontanelle (PF), constipation, jaundice, and mottling. Possible etiologies include:

• ­Dysgenesis is the most common reason.
• ­Abnormal thyroid development somewhere between the base of the tongue and the normal position. Mass would be midline but not cystic. Removal can result in worsening of hypothyroidism. Once a patient is started on Thyroxine, check FT4 and TSH in 1 month.
• ­HASHIMOTO’S THYROIDITIS = CHRONIC LYMPHOCYTIC THYROIDITIS: Hashimoto’s Thyroiditis, or chronic lymphocytic thyroiditis, results in HYPOTHYROIDISM. Labs include +anti-TPO or +anti-thyroglobulin, low FT4 with an elevated TSH. The patient is likely to have a painless, firm GOITER/thyromegaly.
  • MNEMONIC: Hashimoto’s = Hashi-LOW-tos for LOW FT4
  • “Infiltration/inflammation/destruction?” of the gland results in low thyroid hormone (so low FT4), which results in an elevation in TSH. Sometimes, T4 or FT4 may be normal because of the extremely high TSH. A rare complication is transient thyrotoxicosis. Radioactive iodine uptake in Hashi-LOW-tos is LOW. “The thyroid is being destroyed, so how could it take up any iodine?”
• ­­Acquired hypothyroidismis more common than congenital hypothyroidism. Acquired hypothyroidism may present in children as declining growth velocity, delayed puberty, poor school performance, fatigue, constipation, dry skin, and cold intolerance. If a patient is euthyroid up to age 5 years, the IQ should be fine. If the patient is short, it is OKAY to start the patient on meds that can help with catch-up growth.
• PEARL: If TSH is clearly abnormal on the newborn screen (NBS), draw confirmatory labs and start levothyroxine immediately pending lab results to prevent cognitive delays. Treatment initiated after 4 weeks can tremendous cognitive delays. For borderline NBS results, draw confirmatory labs and monitor closely. Treatment can be deferred until results return unless clinical suspicion is high.