Questions and Answers
What is the primary indication of the Pentagastrin stimulation test?
Free T4 and TSH are the best indicators of thyroid function.
True
What happens to T3 and T4 values in patients with increased T4-binding protein?
Elevated T3 or T4, but not free T4 or TSH
Euthyroid with elevated T4-binding protein is due to an increase in ____________________.
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What is the cutoff value for calcitonin in adults?
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Match the following reproductive hormones with their functions:
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In severe hypothyroidism, calcitonin values rise moderately.
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What is the precursor of androgens and estrogens in the testes and ovaries?
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What percentage of testosterone is bound to albumin?
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Only the free form of testosterone can diffuse into the vascular system and interact with target cells.
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What are the functions of testosterone?
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SHBG transports _________________.
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Match the following hormones with their respective primary functions:
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What is the average decline in testosterone levels every decade after age 30?
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Testosterone levels peak at night and fall to their lowest level in the morning.
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CBG delivers ________________________.
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What is the primary application of Triiodothyronine (T3) hormone?
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True or False: Tetraiodothyronine (T4) is a prohormone for T3 production.
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What is the reference value of Triiodothyronine (T3) in children aged 1-14 years old?
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Almost 75-80% of Triiodothyronine (T3) is produced from the tissue deiodination of T4 in many tissues, particularly the ________________________.
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Match the following thyroid hormone binding proteins with their functions:
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What is the effect of elevated Tetraiodothyronine (T4) on TSH secretion?
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True or False: Thyroid hormones affect only synthesis of adipose tissue.
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The amount of serum Tetraiodothyronine (T4) is a good indicator of the ________________________.
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Study Notes
Pentagastrin Stimulation Test
- Used for the diagnosis of Medullary Thyroid Cancer (MTC)
- Procedure: intravenous Pentagastrin (0.5 μg/kg body weight) is given within 30 seconds; blood samples are collected at baseline and 1, 2, 5, and 10 minutes after the start of the infusion
Thyroid Function
- Free T4 and TSH are the best indicators of thyroid function
- Free T3 and T4 are more specific indicators of thyroid function than total hormone measurements
- Abnormal values of total T3 or T4 must be evaluated with TBG measurement
- Patients with increased T4-binding protein have elevated T3 or T4 but not free T4 or TSH
- Euthyroid sick syndrome: low T3 and T4, and normal or increased TSH, but elevated reverse T3
- Euthyroid with elevated T4-binding protein is due to increase in estrogen levels
- Patients with non-thyroidal illness (NTI) have low or low-normal TSH and normal or low-normal T4 but very low T3 during their acute illness
- In severe hypothyroidism, TSH values rise moderately
- Calcitonin is measured by two-site immunometric assays using monoclonal antibodies, and it is also elevated in autoimmune thyroid disorder, hypercalcemia, and all neuroendocrine tumors
- Cutoff value for calcitonin is 10 ng/L (adults)
Reproductive Hormones
Synthesis and Transport of Hormones
- The testes and ovaries produce sex steroids such as androgens and estrogens from cholesterol
- The ovary converts testosterone to estradiol, and androstenedione to estrone
- Peripheral tissues reduce testosterone to dihydrotestosterone (DHT), hydroxylate estradiol to estriol, convert adrenal androgens to testosterone and androgens to estrone and estradiol
- Major transport proteins: sex hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), and albumin
- SHBG transports testosterone and estradiol
- CBG delivers cortisol and corticosterone
- Only the free fraction of the hormone is biologically active
Testosterone
- Principal androgen hormone in the blood
- Synthesized by the Leydig cells of the testis of the male; also derived from progesterone
- Controlled primarily by FSH and LH
- Functions: growth and development of the reproductive system, prostate, and external genitalia
- Physiologic factors:
- Levels demonstrate a circadian pattern and peak at 8 am, fall to their lowest level at 8 pm
- There is a gradual reduction in testosterone after age 30, with an average decline of about 110 ng/dL every decade
- Obesity may cause a decrease in plasma testosterone concentration
- After age 50, men experience a decrease in the secretion rate and concentration of testosterone, and women have an increase in pituitary gonadotropins, especially follicle-stimulating hormone
- Tests for male infertility: semen analysis, testosterone, FSH, and LH
- Reference values: 3.9-7.9 μg/mL (serum)
Thyroid Hormones
- Functions:
- Tissue growth
- Mental development
- Development of the central nervous system
- Elevated heat production
- Control of oxygen consumption
- Energy conservation
- Carbohydrate and protein metabolism
Triiodothyronine (T3)
- Has the most active thyroid hormonal activity
- Almost 75-80% is produced from the tissue deiodination of T4, particularly in the liver
- Principal application: diagnosing T3 thyrotoxicosis
- A better indicator of recovery from hyperthyroidism as well as the recognition of recurrence of hyperthyroidism
- An increase in the plasma level of T3 is the first abnormality seen in cases of thyrotoxicosis
- Reference value: Adult - 60-160 pg/dL or 0.9-2.46 nmol/L, Children 1-14 years old - 105-245 ng/dL or 1.8-3.8 nmol/L
Tetraiodothyronine (T4)
- Principal thyroid hormone product
- Has the major fraction of organic iodine in the circulation
- Prohormone for T3 production
- All circulating T4 originates in the thyroid gland - it is secreted 100% in the thyroid gland
- Amount of serum T4 is a good indicator of the thyroid function
- Elevated T4 causes inhibition of TSH secretion, and vice versa
- Reference value: 5.5-12.5 μg/dL or 71-161 nmol/L (adult), 11.8-22.6 μg/dL or 152-292 nmol/L (neonate)
Thyroid Hormone Binding Proteins
- Thyroxine-Binding Globulin (TBG): transports majority of T4 (affinity for T3 is lower than T4)
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Description
Learn about the role of transport proteins SHBG, CBG, and albumin in binding and transporting hormones in the body. Discover how only free hormones are biologically active.
