OUDE Hypoforum

The last step of the process by which thyroid hormone is secreted from the thyroid gland into blood has not been characterized.

But now, a team of researchers, led by Samuel Refetoff, at the University of Chicago, Chicago, has determined that the protein monocarboxylate transporter 8 (MCT8) is involved in this process.
Although the data were generated in mice, the authors suggest that this explains, in part, why humans deficient in MCT8 have low levels of the thyroid hormone thyroxine (T4) in their blood.

Mice deficient in MCT8 reveal a mechanism
regulating thyroid hormone secretion

1Department of Medicine, University of Chicago, Chicago, Illinois, USA. 2Department of Pathology, Anatomy, and Cell Biology, Thomas Jefferson University,
Philadelphia, Pennsylvania, USA. 3Department of Pediatrics and 4Committee on Genetics, University of Chicago, Chicago, Illinois, USA.

The mechanism of thyroid hormone (TH) secretion from the thyroid gland into blood is unknown.
Humans and mice deficient in monocarboxylate transporter 8 (MCT8) have low serum thyroxine (T4) levels that cannot
be fully explained by increased deiodination.

Here, we have shown that Mct8 is localized at the basolateral
membrane of thyrocytes and that the serum TH concentration is reduced in Mct8-KO mice early after being taken off a treatment that almost completely depleted the thyroid gland of TH. T
hyroid glands in Mct8-KO mice contained more non-thyroglobulin-associated T4 and triiodothyronine than did those in wild-type mice,
independent of deiodination.

In addition, depletion of thyroidal TH content was slower during iodine deficiency.
After administration of 125I, the rate of both its secretion from the thyroid gland and its appearance in the serum as trichloroacetic acid–precipitable radioactivity was greatly reduced in Mct8-KO mice.
Similarly, the secretion of T4 induced by injection of thyrotropin was reduced in Mct8-KO in which endogenous TSH and
T4 were suppressed by administration of triiodothyronine.

To our knowledge, this study is the first to demonstrate
that Mct8 is involved in the secretion of TH from the thyroid gland and contributes, in part, to the low serum T4 level observed in MCT8-deficient patients.

Figure 1
Diagrammatic representation of the steps involved in TH synthesis.
All have been characterized at the molecular level, except
for that involved in TH secretion. The latter, mediated through
putative transporters, is indicated by question marks

Figure 2
The dynamics of rebound of TH synthesis and secretion
after chemical suppression was stopped. Shown
are serum total T4 (A) and total T3 (B) concentrations
and thyroidal non-Tg-T4 (C) and non-Tg-T3 (D) content
(T4 and T3 in the thyroid gland not within the Tg
molecule) at baseline and at 0, 1, and 3 days after
withdrawal of LoI/MMI/ClO4. Data are expressed as
mean ± SEM. At 1 day, serum T4 and T3 levels in
Mct8-KO mice were significantly lower as compared
with those in WT mice (A and B); in contrast, their
thyroid gland content of non-Tg-T4 and Tg-T3 was significantly
higher (C and D). The characteristic thyroid
function test abnormalities of Mct8-KO mice manifested
only on the third day after resumption of the
TH synthesis. **P < 0.01, ***P < 0.001, †P < 0.0001.

Table 1
Intrathyroidal Tg-T4 and Tg-T3 of WT and Mct8-KO mice at 0 and 1 day after withdrawal of LoI/MMI/ClO4

Tg-T4 (ng/mg protein) Tg-T3 (ng/mg protein)

Figure 3
Intrathyroidal expression and localization of the Mct8 protein. (A) Immunoconfocal images from cryosections of thyroid glands prepared from
WT and Mct8-KO mice colabeled with anti-Mct8 antibody (red) and PNA lectin (green). Merged images are shown overlaid on the differential
interference contrast image. Mct8 immunolabeling was detected at the basolateral membrane of thyrocytes (arrow) of WT mice, while no labeling
was detected in thyroid sections from Mct8-KO mice. PNA lectin labeled the thyrocyte plasma membranes in sections from both WT and Mct8-
KO mice. F, follicle. Scale bars: 20 μm. (B) Immunoblot analysis of detergent-soluble protein lysates prepared from thyroid glands of WT and
Mct8-KO mice probed with antibodies to Mct8 and to β-actin as a loading control. Samples from WT mice show a band of 52 kDa, corresponding
to Mct8. This band was absent in samples from Mct8-KO mice.

Figure 4
Histology of WT and Mct8-KO mouse thyroid glands. (A) Low- (left)
and higher-power (right) views of H&E-stained sections from thyroid glands of 14-week-old WT and Mct8-KO mice. Scale bars: 100 μm. (B)
Morphometric analysis of thyroid gland sections showing the number
of cells per follicle, whole follicle area, average thyrocyte size, and colloid-
containing area (for details, see Methods). The data are expressed
relative to WT and presented as mean and variance.

Figure 5
Thyroidal TH content. Non-Tg-T4 (A) and non-Tg-T3 (B)
(T4 and T3 in the thyroid gland not within the Tg molecule),
and Tg-T4 (C) and Tg-T3 (D) (T4 and T3 contained
within the Tg molecule) of WT and Mct8-KO mice 2.6
and 14 weeks old. Bars represent the mean ± SEM.

*P < 0.05, **P < 0.01, ***P < 0.001.

Figure 6 + 7 + 8

Figure 9
Effect of low-iodine diet on thyroid function of WT and Mct8-KO mice. Shown are serum TSH (A), T4 (B), and T3 (C) concentrations. Values are
expressed in absolute amount (bars) and as percentage of the mean baseline value of the corresponding genotype (lines). (D) Effect of TSH
increase produced by low-iodine diet on thyroid gland weight. Data are expressed as mean ± SEM. (E) Low- (top panels) and higher-power
(bottom panels) views of H&E-stained sections from thyroids of WT and Mct8-KO mice after 4 weeks of low-iodine diet. The rise in serum TSH in
both genotypes produced typical histological features of hyperplastic goiter (compare sections from untreated mice, Figure 4A). Note the larger
glands of WT animals at 4 weeks. Scale bars: 500 μm. *P < 0.05, **P < 0.01, ***P < 0.001, †P < 0.0001.

Figure 10
Effect of low-iodine diet on thyroidal TH content
of WT and Mct8-KO mice. Content of
Tg-T4 (A) and Tg-T3 (B) (T4 and T3 contained
within the Tg molecule) and non-Tg-T4
(C) and non-Tg-T3 (D) (T4 and T3 in the
thyroid gland not within the Tg molecule).
Values are expressed in absolute amount
(bars) and percentage of the mean baseline
value of the corresponding genotype
(lines). *P < 0.05, **P < 0.01, #P < 0.001,
†P < 0.0001.

Figure 11
mRNA levels of Mct8 and the other TH transporters

Table 2
mRNA levels of TH transporters in WT mouse thyroid gland

Table 3
Oligonucleotide sequences used for the quantification of mRNAs by real-time PCR