o Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates
o release of thyroid-stimulating hormone (TSH) from the pituitary.
o TSH
... [Show More] stimulates all aspects of thyroid function,
▪ including the release of triiodothyronine (T3) and thyroxine (T4).
▪ T3 and T4 act on the pituitary to suppress further TSH release.
o Diagnosis & Evaluation
▪ What labs are used to diagnose?
▪ Serum TSH determinations are used primarily for screening and diagnosis of hypothyroidism and for monitoring replacement therapy in
hypothyroid patients.
▪ Serum TSH determinations can also distinguish primary hypothyroidism from secondary hypothyroidism. In primary (thyroidal) hypothyroidism, TSH levels are high. However, in secondary hypothyroidism
(hypothyroidism resulting from anterior pituitary dysfunction), TSH levels are low, normal, or even slightly elevated—despite low T3 and T4.
▪ Serum Thyroxine Test
• Testing can measure either total T4 (bound plus free) or free T4. Measurement of free T4 is preferred. The T4 test can monitor
thyroid hormone replacement therapy and screen for thyroid dysfunction. However, in both cases, the size of TSH is chosen.
▪ Serum Triiodothyronine Test
• As with T4, we can measure either total or free T3. Measurement of free T3 is preferred. This test helps diagnose hyperthyroidism.
In this disorder, levels of T3 often rise sooner and to a greater extent than do levels of T4. T3 determinations can also be used to monitor thyroid hormone replacement therapy (all thyroid preparations should increase levels of T3).
Thyroid Test SERUM VALUES
Normal Hypothyroid
Hyperthyroid
Total T4 (µg/dL) 4.5–12.5 Under 4.5 Over 12.5
Free T4 (ng/dL) 0.9–2 Under 0.9 Over 2
Total T3 (ng/dL) 80–220 Under 80 Over 220
Free T3 (pg/dL) 230–620 Under 230 Over 620
TSH (miu/L) 0.3–6 Over 6 Under 0.3
Laboratory studies have different reference ranges. These are relative ranges and are not absolute.
▪ Timeframe for re-check of labs after starting levothyroxine
• 6-8 weeks
▪ Signs and symptoms of hypo and hyperthyroidism
▪ Hyperthyroidism
• Tachycardia
• dysrhythmias and angina
• nervousness
• insomnia
• rapid thought flow
• rapid speech
• Skeletal muscles may weaken and atrophy.
• Metabolic rate is raised
• increased heat production
• increased body temperature
• intolerance to heat
• skin that is warm and moist
• Appetite is increased.
• weight loss occurs
• In addition to thyrotoxicosis, patients with Graves disease often present with exophthalmos. The underlying cause is an immune-
mediated infiltration of the extraocular muscles and orbital fat by lymphocytes, macrophages, plasma cells, mast cells, and mucopolysaccharides.
▪ Hypothyroidism
• Signs and symptoms of hypothyroidism depend on disease severity.
• With mild hypothyroidism, symptoms are subtle and may go unrecognized for what they are.
• In contrast, characteristic signs and symptoms emerge with moderate to severe disease.
• The face is pale, puffy, and expressionless.
• The skin is cold and dry.
• The hair is brittle, and hair loss occurs.
• Heart rate and temperature are lowered.
• The patient may complain of lethargy, fatigue, and intolerance to cold.
• Mentation may be impaired.
• Thyroid enlargement may occur if reduced T3 and T4 promote the excessive release of TSH.
o Treatment
▪ Treatment of thyroid storm
• thyrotoxic crisis can be life-threatening and requires immediate treatment. High doses of potassium iodide or strong iodine
solution are given to suppress thyroid hormone release. Methimazole is shown to suppress thyroid hormone synthesis. A β-blocker is provided to reduce heart rate. Additional measures include sedation, cooling, glucocorticoids, and intravenous (IV) fluids.
▪ Result of not treating hypothyroidism during pregnancy
• Maternal hypothyroidism can result in permanent neuropsychological deficits in the child
▪ Medication to treat symptoms of hyperthyroidism (notice this is treating symptoms and not the hyperthyroidism itself)
• Methimazole
• Methimazole (Tapazole) is the first-line drug for hyperthyroidism. Benefits derive from inhibiting thyroid hormone synthesis.
Methimazole is safer and more convenient than PTU and hence is preferred for most patients—except women who are pregnant or breastfeeding and perhaps patients who are in thyrotoxic crisis.
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